For what I know it absolutely depends where you live and since you live in the UK it probably is UK gallons. Next time can you please ask this question where it would be more appropriate like in Trop. Chit Chat or Beginners questions. Thank you
I usually guess from the number of gallons, assuming standard sized tanks
For example:
55gal = 48x18x15 tank = 45gal uk
you rarely see people quote 4ft tanks as 45 gal
Next size 48x15x12 = 37gal us 31gal uk (ive often seen this rounded to a 40gal, personally i'd round down to 35 gal, ie removing the substrate amount.)
oooh, am acutally getting excited, picking up my 46.7 gallon (US)/ 38.9 gal (UK) tank on sunday. its a 48x15x15. *drools*