I can recommend Ecowagon. We collected our T6 LWB conversion from them last week and they did a fantastic job. We checked out half a dozen converters before choosing them since the quality of their work was a notch above the others. They're about a 2 hour drive from Bristol. Our conversion is Reimo rear elevating roof, rear cabinets fridge, cupboard toilet on one side and hob and sink on the other. Waste and hot water tanks under the flow and cold water tank under the Rib seat. They will do a bespoke design whereas other converters won't. They do a SWB conversion which has sliding rear seats.