In reply to BedRock:
Check your credit score using experian or similar.
Also clarify with them whether they are official refusing you insurance or merely unwilling to quote (this is a major difference) they can refuse to quote for any manner of reason - you fall outside the age bracket they are willing to insure for example, they may have changed their view of the vehicle you are insuring for example (perhaps they've seen bad claims as a result of one of your mods that they have now decided is not worth them insuring). However if they are refusing you insurance then they consider you or the object you are insuring an insurance risk - normally associated with a fraudulent claim, other criminal actions, or insolvency (hence check your credit score)
As stated above. They are obliged to give you all information they hold on you if you do a data subject access request. However they do not have to tell you how they use it or reasons behind decisions made based on it.