Is £540(inc vat) for shower pump and fitting cost too expensive. swindon, Uk?

It’s not necessarily unreasonable, but without knowing the exact pump make and model, the location and access to fit it, and whether a load of other work is necessary such as lifting floorboards or removing tiles and replacing them it’s impossible to give an informed opinion. 

You really have asked “How long is a piece of string?”

If you’re unhappy about the price then get other plumbers to provide quotations for the work.