As an ex-Brit, I'm all too familiar with the weather in the UK - it's one of the reasons I moved some place warmer
For those that haven't experienced it, Britain is an island, so temperatures are very much moderated by the surrounding seas. In summer, highs of 60-70 are common, over 70 and they declare a heat wave, anything over 80 is very rare indeed. A typical winter may see rain on Christmas Day, a white Christmas is not guaranteed by any means! Typical winter temperatures are in the mid 30's, with night time drops into the high 20's. In a really bitter winter, it is possible that the temperatures could drop to single digits, and on northern exposed coastlines, wind chill could take that to negative figures, but again this is pretty rare.
One thing that Britain gets in quantity is rain. Total annual rainfall is nothing to brag about, but it comes as light showers, you know, that gentle sprinkling that soaks through your clothes and chills you to the bone. Not a lot of rain, but it takes so long coming down that it seems like the UK gets around 250 days of rain (or more) each year. Now you know why the weather is a favourite topic in British conversation!
In light of this, I would suggest going with the rolls of fiberglass insulation with underbelly material to protect it from below. Given the lower summer temperatures, air conditioning is rarely used in the UK, so condensation is not a problem (other than the cold water lines, which should be lagged in any case), but humidity is. Using the rolled fiberglass, it is easy to strip a section to replace some rotting wood, and then reinstall the original insulation. Also, the fiberglass will not retain moisture, gravity will take care of any that does get in, if the heating ducts and hot water pipes don't take care of it first. This means there is no wet material nestling alongside the wooden joists and slowly soaking the timber to rotting point.
Hope that is of some help to the OP, and also some help to those that may wish to comment
DaveyB