C150s are powered by Continental O-200s (or the FRA150L/M with the RR O-240) and there is no where to fit a fuel pump.
C152s are fitted with Lycoming O-235 L2C or the later ones, the N2C (basically the same, the N2C having a different combustion chamber design to help prevent lead fouling of the spark plugs - doesn't work and it's slightly less powerful - 108 against 112hp).
The L2C is also used in the Tomahawk. Tomahawks have low wings (and tanks) and have electric and mechanical fuel pumps. The mechanical fuel pump is mounted on the engine accessory casing at the back of the engine. I stand to be corrected, but I've never seen a mechanical pump on the back of a 152 engine in the 152 Parts Manual so it would not be a legit fit unless there was some form of mod in place courtesy of EASA or the CAA for a UK reg aircraft. It certainly isn't mentioned in the Service Manual. Many 152s don't even have the accessory casing fuel pump pad milled out so there isn't hole there to blank!
It may be part of a removed ferry/aux fuel system (in a 152?!) or perhaps someone not very bright has bolted a PA38 engine into a C152! Any pictures bose-x or a registration?