I can either get a 12 gallon nano and have a FOWLR or get a bigger one like 50 gallons but have it as a FO because i dont have a spare few Â£100 laying around for all the LR, or i could just not do one
Nanos are much harder to keep as the water has to be kept much more carefully. toxins build up to high levels very quickly as there is less water to dilute them. If you choose the nano tank be prepared for alot more maintenance and a higher possiblity of a "tank crash" if something goes slightly wrong...which could cost you more in the long run.
As for the 50 gal....I don't have much experience with FO systems and am not up to speed with the filtration you would have to use and what it costs, but it should be easier to keep the water parameters in check and you will have a bigger choice of fish that you can put in there.
What fish are you thinking of keeping?? If there are any specific fish you want to keep tis might make the choice for you as you are quite limited as to what you could keep in a 12gal