Vtech sound-only baby monitor pair with US transformers (not usable on UK voltage, even with an adapter). Very strong signal compared to other brands.
You would need to find two 6V DC power supplies to use these in UK, or use them to replace broken ones (the input voltage is the same for UK and US models).
Baby monitors with US plugs (SE21)
This offer has been gifted to someone and is no longer available.