Just spotted the original thread for the first time...
What happened to that pilot is not unusual, for the UK system.
It can happen to anybody, and continues to happen.
The simplest way to guard against it is by filing a flight level which is
totally obviously decisively in controlled airspace. Hard to give guidelines on what this means (I was told by one ATCO that the rules are in a confidential ATC document) but FL120+ ought to do the job. If you file at lower levels (e.g. FL090 like the OP) then there is a possibility of getting dropped out of CAS, or even the flight plan getting dumped on sight by e.g. London Control.
In Europe, ATC will normally clearly advise traffic asking for shortcuts that one is about to leave CAS. In the UK this often doesn't happen; one can get transferred to "London 124.6" and a foreigner will think nothing of it. A local will know that 124.6 is an FIS service which, in the UK, cannot support an IFR clearance, so actually your IFR clearance has been cancelled without anybody telling you.
It's also an old chestnut when flying from France to the UK where you might be in CAS across France and then transferred to "London 124.6" with the same result. The solution to that one used to be to fly at FL120+ because one is then handled by Paris
Control which has the authority to transfer you to London
Control and your IFR flight continues. In recent years I have seen this work down to FL100, which is an improvement...
I suspect the powers to be decided to do something about it, because having to carry and use oxygen solely to maintain an IFR clearance from France to UK is nuts
Huge amounts of discussion have been done on this on pilot forums. Normally, ATC are less than keen to discuss it, but it continues to catch out foreign piston pilots (or UK ones who have not yet discovered the quirks) filing for non-oxygen altitudes and expecting it to "just work" because the flight plan was accepted by Eurocontrol.
Personally I file for FL120+ and that deals with the issue. One can ask for a "stop climb" if the wx is nice. But there are still parts of the UK (Scotland etc) where the base of CAS is above that, but I have found that Scottish ATC are very much better at managing the situation, whereas in the south if you drop out of CAS (in level flight) they tend to wash their hands of you and won't let you back in. US-style "pop-up" IFR clearances are almost impossible to get in the UK.