If your FAA IR is current and valid you can take the CBMIR route. This is one of the new IR courses introduced in April this year by EASA. If you have more than 50 hours IFR PIC you can take a subset of the CBMIR route which requires no training at all, no need to involve an ATO and no requirement for a recommendation for test.
You can actually book the test directly with flight test bookings at Gatwick.
However, having said all that I would strongly suggest (particularly as you have not flown IFR for a while) that you do at least one flight with an instructor who is experienced at recommending people for the IR skill test. He/she can then tell you whether you should do any more training before attempting the test.