At the end of the day way i see it is -
Can you afford to pay £1800 for the insurance?
Can you afford to pay £3000 excess if worse case scenario you are involved in an accident?
If you are involved in an accident but its clear cut the other party is to blame/responsible then no you would not have to pay excess as you will go through the other party's insurance or instruct a claims management company to pursue on your behalf.
If it is complicated and you need to invlove your own insurance then if the value of the repair is less than £3k then i presume you would pay just the value. However some insurance companies will reuqire you to pay full excess even though the repair works out less.
Insurance can be messy so just be prepared for it should anything happen
If you can answer yes to the above 2 questions and can manage/afford it go for it