Some points from earlier posts need correction or amplification:
1.<<You will also be issued an IMC rating by the CAA for free if you hold a foreign IR. Of course in a N reg aircraft with an FAA IR you are perfectly entitled to fly 'proper' IFR in any airspace in the UK / Europe / Abroad.>>
Not free. £67.
2. FAA regs: VFR on Top is requested by a pilot who is operating on an IFR clearance. If granted, you operate at the appropriate VFR altitudes and are responsible for separation, whilst staying on the IFR flight plan. VFR over the top is permitted without an IR, but one has to maintain the appropriate distances (for the airspace) from cloud, and that would usually mean having a BIG 4000' wide hole to descend through at or before the destination.
3. An IMC rating granted on the back of an FAA IR apparently does have to be revalidated after 2 years, regardless of the currency of the FAA IR.
FTD