I've done Cranfield - Le Touquet airways a few times. What happens in practice does vary a bit. Most usually Cranfield pass you to Luton Approach who give you a direct BPK and a couple of climbs. When you get to around 5000' they pass you to London Control who generally keep you at FL70 and vector you roughly over North Weald and Southend. Once south of the Thames you usually get a climb to FL90 and a DCT TRACA. Nearing the FIR boundary, they usually pass you straight to Lille for a DCT TUKVI or LT and a descent.
Although that's what often happens, it does vary. I usually file the standard route via BPK DET DVR RINTI, but the last time I was taken all the way out towards WCO and cleared to join controlled airspace at FL70 5 miles north of WCO. After that it was DCT CPT and then vectors north of Gatwick towards DVR. Another time Luton couldn't clear me through their airspace, so I routed outside controlled airspace via BKY and BPK, then got passed to Thames who gave and initial climb into controlled airspace and then handed me to London Control. That was better than the other time Luton were busy, so they routed me out to WCO, then back to Woburn and Barkway before passing me to Essex Radar for a tour of Essex.
Those are unusual though - generally Luton do a fantastic job of seamlessly threading us into the system through a pretty direct route. Being slow and poor climbers compared to the rest of the traffic, we must be a pain in the a**e, but they never grumble.
Incidentally, I have taken a really old 172 up to FL100 in the airways. They're not the best climbers, but they do get there. Eventually.