Keep ALL bills receipts, your self employmnet income appears to be substantial most of above all very good advice
However, use of car claim flat rate a bit iffy ~ S/e appears to be main source and not Sch E (PAYE)
Make of car, value new, when bought for how mich how long used for business (depreciation), mileage business/private, petrol oil tax etc may be needed claimed as capital allowances
Take professional advice (see above all good advice) !!