Leaving it on the N-Reg, requires not much. You register it through a trust, there are a couple of firms doing this in the UK. It will cost you between £400 and £800 a year.
The maintenance cost will be a lot cheaper and you don't have to renew your CofA. On an N-Reg the Airworthiness Certificate is valid as long as the aircraft has an annual. So all you have to do is an annual. Of course you'll have to do the manufacturers inspections as well (100hrs) But you don't have to keep a maintenance log and flight log as with a G-Reg.
There are plenty of FAA certified mechanics and inspectors around in the UK who will gladly take your business. (I use one costing me £35 an hour)