As above take expect advise. And from a small business accountatnt. I don't know if Bristol would be too far away, possible not if you're in Chepstow my old accountant was an excellent tax accouant. I was sole trader and later went Ltd, it was very beneficial so don't be put off by it. Just avoid VAT which I'm sure you can.
But when I was self employed my accoutnant relucantly allowed me to put my motorbike against my business as I worked from home, but if I went out to visit clients I'd go on the bike - and I obviously never used the bike for pleasure use ;-)
The correct way I belive, and I think this might actually work out better in the long run is to have it as a personal vehicle and you essentially charge your business-self the for the use of your own private vechicle. You can do this I think by either claiming fuel, for business journeys or the better way is to claim mileage. You can claim at something like 40p a mile which is way above what it costs you, as this is intended to cover not just fuel but a percentage of your running and maintence cost.
Once you've got the vehicle you can put business specic stuff in it and charge 100% to the business, so dog gaurd, seat covers, boot mats, even roof racks if you could legitmatly say it was for dog related business.
Granted it will take longer to cover the cost, but if you wanted a new vehicle and you're goint to be paying for it anyway you'll get money back (or not actually get it back, but reduce your taxable income). And doing it this way you're totally above baord to use it for personal use. Which as you say - you'd use to sleep on the beach, which I presume isn't during the course of business.
This will also continue for each year of ownership and not just the hit in the first year.