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Thanks for your question to History Hub. The plaque marking the location of the "Second Evacuation Unit" appears to refer to the Second Evacuation Hospital, a U.S. Army medical unit formed in 1940 (but dating from World War I) which was stationed in Great Britain between September 1942 and D-Day. According to a unit history compiled online, they were posted in the Whitchurch area outside of Cardiff between February and June of 1944 before departing for the Allied forces concentration area at Henley-on-Thames from which they would eventually embark for France after the D-Day beachhead was established. They appear to have been billeted (temporarily bunked) with private citizens in the Whitchurch area while their semi-permanent barracks were under construction that February and left behind this grove of trees and plaque to express thanks for the hospitality they received during that time.
A cursory search didn't turn up a sign of any of the other two regimental plaques you referred to in your question, but I'm sure a summary search by others will turn them up as well. The unit history link above is pretty thorough, so it should answer most of your questions about that first plaque you referenced. Hope this helps.
Thank you very much for your help I'll check out that link now I'd heard about the evacuation hospital before but didn't realise they were linked