The system over here is way different from that in the US, there you only file for SAR purpose.
Same here (if VFR).
The only difference is that the USA is
one country
The only time that flight plans are mandatory for flights wholly within the UK is when flying a high altitude IFR flight i.e. in controlled airspace, and these are filed via Eurocontrol (
example). You need the full IR for that, too; the IMC Rating is no good because most of the route will be in Class A. The reason one needs a proper filed flight plan for these flights is because the UK doesn't really support the idea of "popup" IFR clearances. In some other European countries things are more relaxed but in general you have to file a flight plan for IFR.