TextBlade has a UK layout, which you can select from a companion smartphone app. It provides instant access to all characters on a standard UK keyboard.
This ALT key feature you mentioned works on both Windows and Mac, and is supported by TextBlade.
4 digit alt codes tend to be cumbersome to type... especially if you use them frequently. TextBlade can make that easier.
MultiMap, a firmware upgrade for TextBlade, allows users to map custom characters to any finger location and layer.
You can map your favorite ALT codes - and get instant access to special characters.
MultiMap (normally $19) is included free with all orders placed before March 1st.
Typed on TextBlade