The fact remains that a number of you folks fly N -reg aircraft within the UK airspace for convenience.
You need to understand a lot more about the European certification and flight crew licensing regime, and then you would have offered a more reserved comment.
The CAA have been asked, in a real live meeting I went to, and the reply was more or less the line already written here, something to do with control and liability. But the speaker would not give details.
FAA IR holders will generally have more experience of GPS approaches because most of them have done their IR in the USA and will have done some over there. Also (generally) the point of an FAA IR (plus N-reg plane) is to be able to fly
IFR around Europe, not just the UK, and elsewhere in Europe there are some GPS approaches which are not limited to VMC. You can look up LKPR for example.
An awful lot of planes have an IFR GPS, usually a GNS430, which has just been screwed in and is not connected to a CDI/HSI. That's no good. A lot of these don't have a current database; it costs US$280 or so p.a. to get a year's worth of updates for mine (KLN94). To fly the approach you have to have the latest database to get the approach in the GPS at all, but anybody not flying European airways has no legal need for a current database. Etc etc etc.
Fuji - I am sure the CAA know how many IMCR and IR holders are valid. AIUI, the IMCR 25-month renewal goes off to the CAA, and the IR 12-month checkride also goes to the CAA (it's done by a CAA examiner, I think). So they know the approx # of legal pilots. The only thing they don't know is how many are medically self-disqualified (e.g. broke a leg, stopped flying, but didn't advise the CAA).
What the CAA doesn't know is how many IMCR or IR holders have access to a plane with a TSO-129 GPS installation, not to mention one with a current database
My guess is that the number will be very low indeed (a few dozen in the UK) and I told the CAA that. I was probably right.
The CAA is rather more selective in the FCL data on their website, and my guess is that they do that to prevent people seeing just how many new PPLs chuck in flying after 1 year, 2 years, etc (most of them, apparently).