Interesting problem. There are a few friendly test airfields on the West Coast of the UK, West Freugh, Warton and Woodford all spring to mind. But, none of them have decent sized crosswind runways.
A possibility would be Valley, a military airfield on the north-west corner of Wales, which has 7,500ft on 14/32, and 4,200ft on 08/26. However, it is a busy training field (Hawks) so you might not be appreciated.
However, I suspect the ideal field (if they'll let you use it) would be Prestwick (EGPK), which is on the West coast of Scotland a short distance South of Glasgow at 55 30.47N, 4 35.13W / 66ft amsl. 9,800 ft on 13/31, 6,000ft on 03/21, a cat-II ILS on 31 and 13 and all the aircraft operating facilities you can eat. The airfield is not at-all busy as international airports go, and there's lots of uncontrolled airspace around there as well. They also have a reputation as being quite amenable to unusual request (how many international airports do you know who accept homebuilt aircraft test flying!)
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