In reply to Ramon Marin:
Hi Ramon,
I work for Orange Campers in Rotherham just outside Sheffield. Drop them a message via the website and you can have a chat about what you need.
I am just a joiner/fitter for the company so don't decide what work we take on, or do any sales so I can't advise on cost, but we can certainly do the simple conversion you're after.
I could do the conversion for you myself at weekends but won't have any time to do it this summer as we're expecting a baby in 6 weeks and I'm trying to get out climbing as much as possible before then!