We have decided the time has come to ditch the tent and get a van. But my partner is 6ft 4. This is going to be a problem . Can anyone suggest a van that is good for tall people ?

Being from Canada, I don't know what already camperized vans would be available to you in UK. What we did here was to buy a stripped contractors van (Ford E250). We had it camperized locally with a pop-top roof, fridge, stove, water system, cupboards and bed. Because we designed it ourselves and had the work done locally we could make the bed and all other parts any size we wanted. This kind of self-design might work best for you. It was also considerably cheaper than buying an already camperized "RV" van and much more to our liking.
If you do it on the cheap as in buy a van and fit it out you can get a bed arrangment that will let you sleep long ways etc, you will then need a small annexe to use so you can sit outside or one of those bug proof tents, it can be done but it is a bit of dicking around, how bad do you like creature comforts, livings van is brilliant, a converted postie van might suit the budget?
Thanks Red Wine, just as I thought ,ie to get what we want "lots of dicking around" Regrettably I haven't that sort of man --- he's too keen to get fishing!

That's what my friend did in the UK. Bought a postal van and converted it. Came out smashing! But you have to be prepared for lots of tinkering to keep things going...

Why would you need to be prepared for lots of tinkering?
Is it because postal vans are old? And you have to tinker with the engine?
We bought a brand new van, stripped of inside finish. Then we delivered it to a fellow who camperizes vans along with a floor plan of what we wanted. We picked our upholstery and inside finish materials. Then we waited 6 weeks for it to be done. It was not cheap (like maybe buying an old van and doing it yourself) but it WAS a lot cheaper than going for one of the well known brand name vans that come all done out already.
Only frustration was that it actually took 8 weeks. But we've had it now for going on 3 years and nothing has broken or needed tinkering with and we do at least one 15,000+ mi trip a year.

I know RVing quite well. My parents were full time for 12+ years and I was full time for almost 2. I've RVed from Fairbanks, Alaska to Puerto Vallarta, Mexico...and back. RVs are moving earthquakes. Things come apart, fall apart, break, all the time due to the rough environment. And yes, the engines will require maintenance also. I'd hate to tell you all the places I had problems. Most in Mexico. My first was a converted Dodge van, my second was a truck and small trailer. My last was a 40' diesel pusher.
I'f you have never had a problem with your van, consider yourself lucky and go buy a lottery ticket. Seriously!!

Maintenance, of course. Every vehicle requires maintenance. And we don't take off on a long trips without thoroughly checking over whatever vehicles and trailers we are taking. Steve takes a tool box equipped with the bits and pieces he might need. We install new tires and whatever else is wearing. We head out with our vehicle in top condition, whatever it is. We've never had any issues with the engine part of our trucks/vans.
Over the years we've dragged a series of tent trailers and then a 35 year old Trillilum (fibreglass shell trailer like a Boler) all over North America - from Prudhoe Bay Alaska to Guatemala. The old trailers required a fair bit of tinkering as we went. You are right - screws work loose. The ancient Trillium we pulled over the topes of Mexico was a particular pain in that regard. No matter what we did to fix the clasps on the cupboard doors, they opened, spilling everything onto the floor. By the turn around point on the Mexico/Guatemala border we had the whole thing bungy corded like a spider web.
So THAT is why we bought a brand new van and had it camperized using plywood not K3 or pariticle board. Over about 60,000 km now nothing has shaken loose or come apart or needed fixing. We've had fuses go and the water tank leaked till we put a new tap on it, but that's been it. It's not a constant case of needing tinkering.
BUT, the lotto max is up to 44 mill so I'll get a ticket today anyway. Who knows? That would fund a few miles.