It is not OK on a UK reg. and obtaining a minor mod. will almost certainly prove expensive and time consuming. I have been there.
I am not saying it cant be done and depending on the installation the hassle may diminish but it is not to be taken on lightly.
If you find someone to just "do it", you risk more problems when the annual is due.
Even "removable" equipment can be an issue unless of course you remove it before the annual!
I wish it were otherwise but better to tell you how I found it.