If you decide on Haverfordwest, I suggest you land at Waterford. They are a customs/immigration airport. You can fax them a copy of your GAR - they don't need it as such, but it has all the information on it that their customs/immigration people will want.
On the way back, you can fill up duty free, provided you are departing on a FPL to the UK.
It's dead easy to find too. Open early to very late, fuel always available and a friendly, "no hassle" place.