Sure, what you want and don't want to do is a most valid
consideration. Personally I'd tend to try to be flexible on a
effectiveness/convenience/cost basis.

Floor cost me £700 to get put in, so any heat savings and cost of
ripping part of it up, and putting it back down, has got to out way
the cost of doing it.

If costs are too much I may as well brick up the air bricks, and
accept that in 20 years time or so, the joists will have rotted, and
replace the lot with a concrete floor.

that's a lot of foam if I understand you right and wouldn't the foam
object formed breach the damp proof course by capillary action?

Doesn't cavity wall foam have the same problem of breaching the damp
proof course ??