It all counts. I had an IMC rating and 50+ hours by sole ref to instruments. All of it was valid towards the FAA IR. I had this confirmed in writing by the FAA in advance in order to prevent any "hiccups".
If getting the FAA IR, make sure you read up on the prerequisites. They are different to those in JAA-land and if you show up for the test without your logbook in order it's a no-go. Also, unlike JAA-land where 95% of the exam content can be core-dumped as soon as you leave the exam room, the FAA test has a very taxing oral exam to complete before you even get in the aircraft so you need to be sharp on the relevant theory stuff on test day.
If you're seriously current on instruments in the UK environment and plan it with sufficient precision it can save you a decent chunk of money to use the FAA IR as a stepping stone. If you're not, it can quite easily cost you far more than just biting the bullet and doing the JAA IR in the first place.