That should be coming soon - according to the
original Navigon press release they are intending to release regional map variants as well as the Europe-wide one that is currently out. It even provided pricing details (in Euros) and said that the regional ones would also have an introductory price (49.99) that would go up to 69.99 in July (implying that they should be out before July - i.e. next few days. For comparison the europe-wide version's current (introductory) price is 79.99 and supposedly goes up to 99.99 from July.
It looks like they were planning to release a UK variant and a (Germany, Austria, Switzerland) variant first and then others after that - so hopefully at least those two will be out before July anyway.
Considering how the pricing relates to the current £ price for the Europe-wide version it looks like we could expect the UK-region version to be about £35-40 if released before July, and ~£50 from July onwards.
If it's not out by the end of the month it will probably be worth just getting the Europe-wide version while it is still £55 and then doing the unzipping trick on the downloaded file to just extract the UK bits if you just want those on your phone - at least then you will have all the other maps saved on your computer should you want them later. Otherwise you will be paying £50 anyway and not have that option.