There is a speed limit. I believe it's 60mph
Don't GLASS, TRF and CRAG recommend a maximum of 15mph though?
TBH I've not often seen a 4x4 travelling at any speed faster than 15mph on a lane, but almost all bikes I've seen are doing what appears to be 30mph+.
When I have seen 4x4's doing so, it has been on lanes with very good surfaces and out in open fields where visibility is excellent.
I have to agree though that green lanes should have speed limits set on them. Even if all were set to 20mph by a rule that says unsealed roads are subject to a maximum of. That way, councils would not need to go erecting speed limit signs on most lanes as they would fall under the national speed limits of the preceding sealed surface road. Only ones on lower limit roads would need signs (which would be the minority of lanes) . Also local councils could then choose to introduce lower limits on some lanes to help protect them from potential damage.