Crossing an international FIR boundary without having filed a flight plan can produce some surprises such as a fighter jet formating alongside you and the pilot gesticulating to you and its unlikely to be a friendly wave!
1. You should immediately change to what frequency?
2. You do have on board a set of INTERCEPTION SIGNALS don’t you? (required by law to be carried on every flight, although the chance of being shot down in the circuit at Halfpenny Green are fairly remote even though the width of some pilots circuits probably do need a flight plan)
So what’s your best guess? if you failed to follow the fighter jet pilots instructions would he shoot you down? Don’t think I would like to find out and in some countries I think it might be more, shoot first, ask questions later!
While I was instructing at Aberdeen one of the pilots next door in the Air Taxi company came back to Aberdeen one night from the continent without filing a flight plan and as he approached the coast two English Electric Lightning jets formated on him in cloud! Unfortunately one of them actually collided with the Aztec, luckily this was more of a graze than a collision and both pilots landed safely afterwards!
This pilot was not so lucky, note the phrase “was probably shot down”
A flight plan must be filed to cross the UK FIR boundary, so that does include going to Jersey and the Isle of Man
You are recommended to file a flight plan when flying more than 10nm from the coast over water