Many historic venues are currently in dire times, already up against funding cuts, now closed and making no money, and facing large expenses for clean up, and then restoration. For example, Yorvik Viking Centre found themselves with 50cm of water, had a disaster plan that they were able to quickly and effectively put in place to protect the historic artifacts, but are facing some very long term closures which will require expensive work. Leeds Industrial Museum enacted their disaster plan, having put measures in place after previous floods. Sadly the water rose higher than possibly expected, three times more than the previous recorded height. Other venues have been doubly hit, with the venue damaged, and staff made homeless at the same time - rendering them unable to give as much assistance as is needed.
You can help flooded venues by volunteering time to help clear silt and other damage, or by offering to brew up while those with the expertise do the work. You can support venues and communities by still visiting - check ahead and you may find that there are areas and venues still open or recently re-opened - don't assume everything is devastated. Many venues are now setting up online donation boxes, but even if they aren't able to do this, you could always contact your favourite stricken venue and then pop a cheque in the post.