regulations

The wiring will certainly not comply, arguably the plumbing, although not standard, would not present any hazard greater than that in a system allowed under French regulations.

If you state that the plumbing and electrical work are not to french standards in the compromis de vente, you will have avoided making a vice caché and should not have any further comeback.

UK cable is different. The earth conductor is often smaller section than the live and neutral and is not double insulated for starters. There is not a european equivalent for standard UK flat cable. Consumer units are also to a much different spec. In short nothing much is the same, certainly with regards to the regulations fifteen years ago.

An open vented heating system is perfectly acceptable, and in the case of a solid fuel boiler, is certainly safer because it relies on fewer mechanical safety devices.

As in the UK though, with solid fuel appliances, a galvanized steel expansion tank should be used, not a plastic one.