I want a ‘Transit’ style or similar big enough for 2 bikes in the back with room for a bed above them for sneaky camping.
Needs to be:
Economical
Reliable
Easy to drive on motorways.
Max £3k, don’t mind older vehicles as long as reliable.

Actual real-world owner experiences would be handy along with actual mpg etc…

To get a bed above 2 bikes and access them easily your looking at Med High Transit

IMG_7588 by Mike Smith 79, on Flickr
That setup took trail bikes with front wheel off stood up and enough room to sleep comfortably. A lower roof and the bikes are going to be lying down. There was a slot on the side for the DH bike…

Top tip is get the interior space for some vans and mark it out in the house and see what you can fit in bed/storage wise. We had a lower bed to start but wasted loads of space that way.

A standard height van won’t leave you much room above for a bed (depends if you want your bikes lying down or upright). The van pictured in the above post is a bit taller.
I recommend the Vauxhall Vivaro van as a good choice. I’ve got an old battered SWB one and it’s been one of the best I’ve ever had (mileage isn’t to bad either if you drive them steady). Transits are good but tend to be a bit thirsty compared to Vauxhall/Nissan/Renault (same van, same factory different badge).

It’s Med Height, just enough to stand up straight in for me and to sleep in without knocking myself out waking up suddenly.
In that configuration we could get 4 bikes in easy (up to 7) and the stove, gas BBQ, tools, bike stand, portable pressure washer and tool box all under the bed.

I’d also suggest looking at awnings to sit under when you need to get out of the van but the weather is less than awesome.