As you are still keeping an eye on this thread and for the benefit of others here is my personal recommendations. I own lots (but not all) DLC and here are some of my favourites . . .
Now, Rudolf makes a good point regarding bang for buck seen as we are only allowed 4 DLC packs a year in this case. Regardless of your DLC "limit" or anyone else's limit for that matter you have not picked a cheap hobby and you are never going to have enough money to buy all the rolling stock you want. So my recommendation is to fight off all impulse buys and shop wisely and smart. Always hold off until the Steam sales when everything is half price to double that DLC limit to eight - 4 in the summer, 4 at Xmas. The thing to note as well is that Steam is not the only place to buy DLC for TS2014 and the other places to buy from all have sales at different times so keep checking the sites regularly and signup for newsletters if possible. Also certain rolling is not available on Steam.
List of places to buy TS2014 rolling stock; I have bought good quality DLC from all of them:
Steam
Fastline Simulation
Digital Traction
Armstrong Powerhouse
Just Trains
Steam Sounds Supreme
Dick Cowan
http://www.dickyjim.com/railworks.html
Check the Railsimutor.com site every day as there is generally around %30 discount on a different DLC pack each day, and a weekly deal on another. Sign up for the Just Trains newsletter for their deals.
Whilst browsing through the Steam store, look out for the bundle offers. For example, The Portsmouth Direct Line Collection on Steam bundles the Portsmouth Direct Line Route (and Class 450 EMU), BR Class 421 '4CIG' pack, Class 455 pack, South West Trains Class 444 pack, BR Class 423 ‘4VEP’ pack and the BR Class 33 pack for a discounted £59.99 (from £85). I bought this bundle pack for half price - £30. A bargain for a route, 5 EMU's and a locomotive in 7 liveries and with ballast wagons. I'm pretty sure all these bundle packs are discounted %50 during the sales. Advise your parents that you get the models cheaper if you buy in bulk and wait for the sales.
Now looking at individual DLC I have for bang for buck factor AND taking into consideration that the OP's interest is in UK trains then my first recommendation would be the European Loco & Asset pack. You need this if you came to the simulator after version TS2012, otherwise you already the locomotives in this pack. That pack gives you 4 iconic British Rail locomotives - Class 37, Class 47, Class 55 and the HST. Although older models now lacking the latest effects such as rain on window, the HST, with Armstrong Valenta sound pack applied and headphone volume on high, the HST is still a very enjoyable drive. These locos are all at home to almost all UK routes released in TS2014. To enhance the bang-for buck factor of this pack these loco's all come in BR blue as standard (unlike a lot of the latest UK DLC pack releases it seems) and there are numerous additional free re-skins in the UKTS download section that covers almost every single livery variation ever adorned by these locomotives. You also get Mk1 Mk2 and Mk3 carriages and a variety of UK freight wagons and the Black 5 and 7F steam locomotives too. There are lots of re-skins on the UKTS site for these also. A selection of German locomotives are also included. You could also decide to go for the East Coast Main Line route addon instead and sacrifice the class 37 and German locomotives for the Newcastle-York route (you still get the Class 47, Class 55 & HST with this pack)
I don't know if this is the case anymore but the Oovee HST buffer pack comes packaged with the WCML-N route so buying this route may also be a route to owning the HST (albeit the buffer version). Free re-skins available as well for the HST buffer pack including the Network Rail measurement train. With the WCML-N route not only are you getting one of the larger routes available for TS2014 & possibly the HST buffer pack but you also get the Class 86 electric locomotive in Intercity Swallow and locomotive hauled versions of the Mk 3 coaches. Reskins available for the WCML-N Class 86 and coaches are available for download here also.
All these aforementioned models can be enhanced with sound pack from Armstrong Powerhouse which I recommend. If you buy these packs you will know what exactly what it sounds like to be on or near the British rail network There are links to YouTube clips so you can hear what the packs do before you buy.
There are also other instances of buying one pack automatically giving you another pack free. For example you get the Norfolk Southern SD40-2 High Nose pack free with the Ohio Steel Route. The Edinburgh-Glasgow route includes the Class 170 pack. Glasgow Airport Route includes the Class 380 pack. I'm sure the TEA wagon pack is also bundled with another DLC pack - possibly the new Class 66 packs.
Now of more recent UK models released I have found the DLC below are among the most enjoyable to drive because they have good sound sets as standard and have advanced scripting and advanced cabs with more buttons to press and the model has uses textures and use the latest effects. They are generally higher quality models.
Class 57 available from Steam.
Class 150 from Steam
Voyager Advanced from Just Trains
Class 90 + DVT from Armstrong powerhouse
Another essential UK DLC pack to have (in my humble opinion) is the Class 20 Collection available at Just Trains. It is an older model with no rain effects on windows and maybe a little expensive but there are 3 Class 20 variants (Disc, Headcode and refurbished examples) in 10 liveries and comes with FEA Rail Head Treatment wagons and FNA Nuclear Flask Carrier wagons - these are very nice wagons indeed BTW.
Hope this helps you build up your collection!