In SE England you would be looking at £250 a month to park, landing fees £10-£15 and AVGAS £1.45 per litre. Instrument approaches normally have to be paid extra.
While you would be legal to fly in the UK (and IFR/airways with your Nreg/FAA IR) I would suggest that you get some extra tuition on UK rules and procedures. The CAA can get quite unpleasant (£££) if they have to chase you for some procedural transgression which wouldn't have applied at home.
Do let us know if you manage to get your Mooney across here, and good luck!