Hi Frazer.
well done for making the best decision of all, already; that is, 'to investing start young'. this is the most crucial of all tips. over time, (ironically, very apt as a cliche), you will benefit in folds by starting investing so young. so much so that retiring at 40 or even younger is very much on the cards, having started at 19. I bought my first property at 19 so know it is not going to be easy for you and at times you will think, "why on earth did i tie myself down like this". But life is never easy and when those feelings pass, you will look back and thank your lucky stars for making what is likely to be one of your wisest ever decisions at this young age.
that said; think about buying the house for you to live in. then rent out the spare rooms (so, get in lodgers, rather than tenants). you'll end up likely better off as you get tax breaks for renting out rooms and you are very much in control. it should mean that you are living for free and with a bit of spare cash after the mortgage is paid. more importantly, that way, you can side step the 'how do I get a buy-to-let mortgage with a low income. I am assuming your income is low just because the £25k limit is an issue you have mentioned.
Once settled into the house and having rented it out for a couple of years, you should be able to demonstrate higher income, by way of pay rises/promotions as well as the rental receipts from your lodgers. This, coupled with the fact you are a home owner/occupier, will mean more lenders willing to accommodate you for your first BTL.
Good luck and don't be fully put off by DSS or HMOs.......there is good money to be made there; just more work involved........but then when did higher returns ever come without a down side.
Let me know how you are doing at 40.