I don't know about using the points as I put them all into airline miles but I usually stay 50-60 nights a year in UK Marriotts so here's my thoughts on the good and the bad....(from the point of view of someone working for a day or two in the area, rather than going for a holiday break)

Favourite: Tudor Park Maidstone; Bournemouth; Hollins Hall Bradford; Dalmahoy Edinburgh; Liverpool city centre; London Maida Vale and County Hall; Renaissance Chancery Court; Manchester Victoria and Albert; Manchester Worsley Park; Preston; Cardiff
Marble Arch: OK location but rooms generally small and many in need of refurb;

Least Favourite and would always avoid:
London Regents Park and Heathrow Renaissance;

Stayed at many others but these are the ones that stand out as better or worse experiences and locations for me. PM me if you want anything else