If you claim 'mileage rates' against your tax (rather than recording actual expenditure on fuel and other motoring expenses) you can't count your vehicle insurance as an additional expense. (The 'mileage' system assumes that the mileage rates include all motoring costs, except the cost of a loan to buy the vehicle).
If you don't use the 'mileage rates' system (but, instead, claim your actual motoring expenditure) you can claim the full amount of vehicle insurance if the vehicle is solely used for business purposes, or a 'pro rata' amount if it's used for both business and personal use.
Chris