you will find that if you spend a bit of time and not much money making your shed secure it will be easier to get insurance - I have a ground anchor rigged up to padlock bikes to, lined the walls (up to 4ft) and door in 18mm plywood, fitted a simple (cheap) alarm to the door, and fitted extra hinges and hasps/locks. when you list this all to insurance companies they seem to accept that it is more of a 'garage' than a shed. and this makes a difference. Unfortunatly a lot of it still goes on your postcode which can be a bit unfair.
Carol Nash does f+t insurance for garaged bikes not used on the road which can, i think, include insurance while on a trailer.