Hello Stuart,
Welcome to the Forum.
You cannot change the language.
The sat nav won't work either, it won't be able to locate the satellite.
Even if the disc system works, you will need UK discs, and I don't think any were ever available.
The DVD size is, probably, for CDs (and maybe DVDs, the smaller one is probably for minidiscs.
Both should work, but may require region 2 DVDs (my later model does not).
If you go the new head-unit route, as many do, proceed with caution paying particular attention to steering wheel functions, including cameras (if you have them.
There is a ton of info on Head-units within this Forum.
Good Luck