As has been said, your issues really are getting insurance, the potential repeated costs of repair if your insurance won't cover flood damage and the difficulty in selling on the property.
The best way to ascertain the actual risk and likelihood of flooding is to look at the flood map jam bo linked to and speak with the EA about the risk to that particular area. You can put in an information request to the EA for about £50 and ask, what was the flood level from the lastest 20 years of flooding known in that area (and what return period of event was it estimated to be) and what are the 0.1% per annum (10 year return period) to 0.01% per annum event (100 year return period) flood levels modelled for that location. That should give you an idea of how often it might flood and how bad it could be. i.e. if the last flooding event was a big one (estimated at 70-100 year return period) then thats about the worst flooding you might expect in your lifetime (though a bigger event could happen, its very unlikely). If however the last flood event was a 1-in-10 year event, then its likely to flood again 2-4 times in the next 30 years and one of those events might be much more substantial. (That very much depends on the character of the river and its catchment and the connection with surface water drains and the like in the vicinity of the house).
There is loads of information and different structures on flood protection you can buy to hold low-level flooding back. Everything from removable covers for doors and air bricks to the full refit jam bo described.
Depends how much you want the house but I would at least get better informed and cost up the appropriate flood protection + risk of a bigger event before you exchange! (Note that the removable door covers will work up to about 30cm but more than that and it's best to allow the building to flood for structural reasons).