I used to have a PA28-161 that had spats, which I think were the smaller ones. I'd agree with the 3-4 knot decrement in cruise at about 70% power.
Vaguely close to LCY I'd try North Weald (North Weald Flying Group are nice folks and have a suitable aeroplane), Fairoaks and Booker.
If you get sufficiently desperate, drop me a line. I don't presently have access to a -161, but do have access to a -151 with spats at an airfield in the Midlands. I don't know how similar the fuselage and undercarriage are between that and the Archer however, I do know that the wings are different.
G