I think you need to look at your question and dump all the guff, leaving yourself with fact.
First the police can stop any vehicle on a road anytime they like to check whether the driver has a driving licence or/and insurance. (RT Act)
The Form HORT1 is a 'producer' that gives you time to find the relevant documents and take them to the police station you chose for inspection.
If the documents you produce are valid then no further action. It is therefore obvious that unless you can produce a certificate (cover note) of insurance for you to drive that vehicle that date and time - you will probably be prosecuted.
If you cannot produce a full driving licence that allows you, wherever you hail from to drive on UK roads, then again you are likely to be prosecuted. If you produce a Polish EU licence ok or whatever.
In essence you either have the proper cover or you don't.
A letter from anyone is not enough. All they wish to see is a Kosher pair of documents. If not, it's the gallows for you.