Is 75mph legal on a motorway in the UK?

I know it's 70mph on a motorway or a double dual lane carriage way, it's 60 on a single dual carriage way, but I do 65mph on a single and on a double or motorway I do 75mph so will the police be bothered stopping me then? I have a 2004 Ford Focus ghia mk1 with 172,000 miles on the clock and beneath my car their is a rumbling sound I think it needs repairing any suggestions what it could be? Could it be the exghuast or something?

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