If you are passing nearby, you may be lucky and get a great service, or you may be unlucky.
I can understand this point to a degree. Even with VFR Flight Following over here we can get dumped or not handed off if the controller is too busy with their IFR/inbounds, etc.
No issue here if you have to focus on certain traffic.
But I think the OP was referring to
en-route services forpopup IFR flights.
In this case we (in the US) will never be dumped because we would
file a IFR plan mid-air and then put onto IFR frequencies to get a continuous service for the rest of the way. Note that workload goes up quite a bit in the cockpit when manually flown in IMC which explains the OP's 'stress'. The last thing pilots want to face is being blocked by controlled airspace coming down or squeezing us into narrow gaps while the ground is going up.
Soaringhigh, you do realise you should be aiming at the CAA rather than NATS, don't you?
Maybe! But I'm based in the US so I think it'll be up for UK based pilots to do this.