I reckon if you get a system where, say, 6-8 constituencies are merged into one, and then vote for the same number of MPs as the number of constituencies merged, you'd avoid any serious issues with cockblocking. 6-8 constituencies in the same geographic area would have largely similar populations with similar voting patterns, especially if care is taken during the merging phase to group them well, so no party could cockblock the entire constituency. The MPs would then represent the whole new, larger constituency, so that anyone living in that constituency can deal with an MP of the party of their choice, rather than having a specific MP assigned to a specific town.
And maybe it's just because I live in a rural area where I've got to travel across constituency lines to get to many amenities, so I'm used to considering a fairly wide geographic area to be my "local" area... but I really wouldn't care if I had to travel 15 miles to see a Labour MP, rather than 5 miles to see a Tory one. The town 15 miles away has all the same problems as the one 5 miles away, so it's not like the Labour MP wouldn't "get" it if I went to them saying "hey, I've got this problem going on, can you help?"