Fuel required depends not just on aircraft type, its load factor, diversion alternate, holding fuel taken due expected delays (statistical delays can help assess this)...
For 737 series the calculation can be roughly made as:
- trip burn 2.5t/(first hour of flight, GLA-LHR is around that so thus valid assumption)
- final reserve 1.2t (30 mins)
- diversion fuel 1.2t (many available choices within 30 mins like LTN, LGW, STN, BHX, EMA, BRS, ...)
- contingency 5% trip /w minimum 200kg
- taxi out fuel 100-200kg depending operator
- holding fuel (as required)