theres a product from meguires. it takes out scratches from headlamps. in the demo vid the guy is using a rotary mop. google it
that's plastix I think... works on plastics.. and not very well either imo!
however... polishing glass is a minefield... you need special glass polishing pads.. and the right cutting compounds.. even then you stand a 1 in 10 chance of the glass shattering due to excessive heat build up from the polishing machine
even IF you did manage to polish out the scratch you would still be left with some kind of distortion due to the level of the glass in the polished area being lower than the unpolished glass
as for an MOT fail... if it is within the drivers line of vision then it might well be... but it depends on the placement. do you have fully comp insurance? it might be worth speaking to autoglass?