You should use vapour barrier tape (e.g. alu tape) over the gap between insulation and wood to elimitate moisture migration from the inside. On top, on the other hand, the more vapour open the construction is, the better. Full timber floor boards wil be ok for this. OSB would not be ok (it cannot absorb or transport moisture).

If the roof on top is insulated, the moisture problem is already largely mitigated, but a vapor barrier from the inside is always advisable in the UK climate.