Don't know about the legal position but I considered this before I fitted my own boiler and went ahead anyway. ISTR that if you're involved in a car accident and have no MOT that is not enough for the insurance to refuse to pay ( I could be wrong here! ). They must prove that the car accident was caused by a defect that the MOT would have found. Inother words, I expect your insurance will only be void if it was your installation that was defective, since by definition, if you screw up, you aren't competent.
How sure are you of your competence? My background is not gas-fitting but as an electronic engineer I reckoned I had enough nous to take on the job. If you do it, make sure you do your reseach, i.e. correct positionig of boiler, ditto for flue, leak testing with a u-tube manometer ( following the prescribed procedure as listed , I think, somewhere on uk.d-i-y by Ed Sirrett ), leak testing with the appropriate fluid ( not squeezy for corrosion concerns I think ), mking good solder joints ( the secret IMO is to scour both the pipe AND the inside of the fitting before soldering with flux ), removing excess flux to discourage corrosion, making sure the gas pipe is only passed through a cavity wall whilst being in a length of plastic pipe ( sealed to the pipe at the appropriate end ), to prevent a gas leak blowing your walls apart, making sure your boiler is wired to the appropriate regs ( DP isolation switch, thermostat in new installations etc ), and so on and so forth. It's pretty much all on the web somewhere, a good diy book helps with things, as does the boiler installation instructions. It's your call.
Andy