Just got back from a 5 day jaunt to Japan visiting the Fujimi Panarama MTB park (http://www.fujimipanorama.com/mtb/)and I have to say we had a blast. Admittedly my group came from somewhere a lot closer than the UK, but other than flights I think that all we paid each, for a group of 7, was approx.:
Van Hire, Gas and accommodation (2xchalets 5 minute drive from the park) = 250GBP
Daily unlimited Park access (inc Gondola service) = 32GBP
Lunch at the park canteen = 10GBP
If your visiting Japan on vacation or work you can hire bikes (all types) from a bike shop located in the park (http://www.backyardgaragehouse.com/), better to contact them via email as there’s not many English speakers working there.

This is a video of the C course (the easiest) to give you an idea of the riding, admittedly the B and A course are far harder but I had limited access due to a local competition taking place this weekend (didn’t want to slow down the Pro’s practising).
[video]http://youtu.be/8E2rDAfR-k8[/video]
If anyone has any thoughts of visiting please feel free to email me and I will give you more information regarding accommodation, bike hire, general getting about etc.