Opella Cistern Bottom Entry Filling Valve Adjustable Arm 1/2" with Ball Float
Range from the cheapest replacement of a toilet float valve £25 (MJM Plumbing & Property Maintenance based in SW1V) to the dearest replacement of a float valve £80 (AP UK PLUMBING based in SW8).
Comparing plumbers estimates for replacing a toilet float valve in the cistern tank gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - Get rates and quotes to replace a toilet float valve in SE11 by posting a job with HaMuch and we will send it to plumbers in SE11.

"Replace float valve in toilet cistern as well as the other parts in the water tank. Rubber gasket (80mm) is old with limescale and disallows water from filling the tank."
SE1