Some very good advice on here already, but my take is this...
Sign the form, send it back, pay your fine and forget about it.
They have the right to follow and stop any car they deem fit in the interest of public safety and crime prevention. If you were doing nothing wrong they could still stop you if they wanted to, it was late at night you might have been heading home from a pub or club etc.. It is, at the end if the day, their job. Traffic Police are quota'd to pull 300 cars per month for spot checks, regardless of them saying they don't have quotas, they do. It may have been a case that they were always going to pull you but you may or may not have sped in that time and hence they slapped you with a ticket...
VASCAR, cannot be operated by the driver and so if it has been used and there wasn't two officers present then you could argue it in court as he would be driving without due care and attention. They do not have to provide ANY physical evidence of a driving offence, they do not need to have you on film or video, they do not need to have you on a handgun or anything else. They can use their own speedo to make a very close guesstimate of your speed by following you between any two points from a set distance behind you. VASCAR needs 400M to calculate the data that a good officer could calculate in his head in half of that. It isn't hard to do. They do NOT need to prove the calibration of the speedo in their car, when was the last time the one in your own car was calibrated? Exactly. It doesn't happen. Although rest assured their car will have been checked for accuracy within the last year or so.
I've been there, I took it to court and they gave me 4 Points and £100 fine for something they technically couldn't prove. The problem was, I couldn't prove I wasn't speeding and he had eleventy-thousand years of experience m'lord. In truth, I probably was speeding albeit not by the amount they tried to say but still speeding is speeding. Much like you, it was a 40MPH road that turns into a dual carriageway (50MPH), I saw him at least three cars behind me in traffic and he was clearly looking at my car as he'd pulled out slightly from the line of traffic. I accelerated and he'd overtook the cars behind and me before I knew it and the lights came on. I was stopped for no insurance but on providing him with my insurance certificate he told me "he had to accelerate to catch up to me" which meant I must have been speeding. My argument was the same as yours, of course he had to accelerate if he was behind me, that's sort of how it works to catch up to somebody.
It isn't worth the aggro', pay the fine and think no more about it.