Dear Wolf,
If you can keep up this new lifestyle, you will do very well! The ability to live frugally can make you rich very quickly.
I agree with others: the buy-to-let thing is over, and I doubt that you will make the kind of money that the previous generation did. Some people made a lot of money because they made significant capital gains on leveraged investments (borrowed money). That won't happen to you; house prices are now at an unsustainable high. At best, future growth will simply keep up with inflation; at worst, you'll lose everything. The actual income returns on house letting are pretty modest. My landlord rents out this £600K house (2 flats) for £2000 a month. That's a 4% return on his capital. You can do much better than that just by buying an index ETF. The ETF won't call you up at 3 am because of a burst water pipe. Many people go into buy-to-let because they understand it (or at least they think they do) and shy away from stock/gilts because such things seem too complicated/mysterious.
On the other hand, renting out a spare bedroom in your primary residence is certainly a frugal (and tax-efficient) thing to do.
My advice is to stash your savings in an ISA and buy some sensible low-fee index ETFs (e.g., VOO, VTI, QQQ). Do NOT get the kind of stocks and shares ISA where they manage the money for you in high-fee unit trusts (mutual funds). On the other hand, do not buy and trade individual stocks. Seriously, just get a broad stock-market ETF and keep adding to it in your ISA every month. Don't panic about the ups and downs. You'll need to read up on this stuff, but it doesn't have to be complicated.
The most lucrative investment for you, however, is in yourself. Right now, you are earning minimum wage. You need to develop some skills that would enable you to get a higher salary. Learn how to code (if only something like javascript and php for website development). If that's not your thing, and you are really interested in property, then learn a building trade (plumbing, electrical, plastering). Maybe you have a passion (cooking, music, photography) that could earn money with a bit more formal training. Your local college has programs for you and, depending upon your age, it might even be free. If not, spending money on this could still be a very worthwhile investment.
Also, regardless of what you go into, I would advise you to develop your writing skills. Learn what a run-on sentence is and how to avoid it. Your writing style projects your competence and authority; good writing can open doors and ward off trouble. Finally, get seriously up-to-speed with excel and basic math for finance. I find that many people, even university graduates, are barely literate and practically innumerate. It doesn't take much to leave most of the competition in the dust...
Good Luck!