The government came to a new agreement with the Association of British Insurers last month about providing flood insurance, have a Google on " flood re" (on phone, have failed to do link to this twice!). You should be able to get insurance. It will be pricier, but it's also now capped. Having said that, whether new owners get insurance is a grey area.
The insurers are asking you if your property has ever flooded because they need to assess the risk of this happening again, a bit like asking a driver if they have ever had an accident. Insurers also refer to the Environment Agency flood map, of properties which have the potential to flood ie, those that do not benefit from.defences or where they are defended, if the defence fails, and assimilate all this info into your premium. You can see this map too on the EA website, there a "Flood" clicky on the front page.
You may need a letter saying that there is no record of flooding at your property since the Thames Barrier was completed, which you can get from the Customers and Engagement team at your local Environment Agency Area Office (EA website, or pm me your new address and I can look it up for you at work).
If you still have difficulties, try contacting the National Flood Forum.
I realise this message would be a lot more helpful with links, but my phone is not.playing!
Rightio, off to work to see some flood defences bei.g built - guess who.I work for ?