Two suggestions:
Nissan NV200 is a good shout as the floor is lower than most (so a tall MX type bike will fit in). Not quite long enough to get a lot of bikes in straight but will easily fit at a diagonal. 1.5 litre Renault engine - I'd go for the more powerful version which has 6 gears.
Can be had with rear seats and windows but they tend to be ex-taxis.
Not wishing to teach you to suck eggs, but you do know about reduced speed limits for vans. (Not that anyone takes any notice).
My solution was to buy a wheelchair access vehicle. A Fiat Doblo. Ramp for wheeling the bike in and out. It (just) fits with the passenger seat in, although I do take mine out for convenience. 4 seats for normal day to day use. Get 2 builders bags of garden waste in the back for the tip and can get in without a permit as it's classed as a car. Likewise, no van speed limits. Just wish it wasn't a Fiat
Next time (in maybe 12 months) it will be a VW Caddy Maxi - but the rear bench will have to come out to get a bike in.
HTH
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