Ouch, that could get pretty steep when it comes to motorways. I don't have a huge problem with proportional fines necessarily, but I'd be hugely annoyed if I got fined 50% of a weeks wage for doing 71mph on the motorway. If the fines started at the supposed 10% + 1mph limit, that's a bit different (perhaps they'll do that anyway).
Doing 45 mph in a 30 zone is quite different though...