If you want a specialist photographic policy, check out Glover and Howe. My comments relate to their cover for amateurs - things are different if you're a professional.
Glover and Howe were recommended to me, and their policy seems fair enough. Locked boots are covered (and they will cover equipment under the load cover of an estate or people carrier) - the only requirement is that there must have been forcible entry. You can't leave equipment in the car overnight (between 2100 and 0600) unless it's in a locked compound or locked garage.
Equipment is insured for full replacement, which is different to most insurance policies where you only get the market value. As stated in my certificate:
Basis of settlement: FULL REPLACEMENT - the cost of repairing or replacing an item of the insured equipment or property as new or at Insurers option making a cash settlement without any deduction for wear, tear or betterment, provided settlement does not exceed the sum insured stated and agreed in the schedule.
This means that it's going to be more expensive than adding to your household contents insurance, where you'll just get the market value of the item. If you have a lens stolen, for example, you get a new lens - or the nearest replacement / equivalent if that lens is no longer available. You are warned in the proposal to make sure that the values you've stated are sufficient to replace the items - including inflation if you wish.
The premium depends on the total value insured. Items over £100 have to be itemised, but there's no limit (except that the maximum cover for equipment in an unattended car is £6500, and premiums for over £10000 are on request).
Details of the excess and no claims discount should be noted.
I believe E&L are the company to avoid - there's various horror stories around the Internet.
David