You can't offer a back door without risk that others will use it.

If you produce messages that can be read by two keys, the recipient or key XYZ which is held by the UK government then anyone who gets that XYZ key can decrypt every message.

On top of that, the politics of back doors are just too problematic. If you give the UK a back door then Russia can come to you and demand one too. Any government, by establishing a precedent that they get a back door opens up to services giving access to their enemies too.