Well as I have flown my wetlease BMI LX flight (GVA-LHR), I thought a mini-TR might be appropriate.
The flight is handled as a regular LX flight. It's really only as you board the plane that you realize the difference. The plane was a 320, clean and we were pleasantly greeted on boarding by the BMI staff. One of the FAs spoke good enough French to make announcements without reading from the script, all the others English only. Seats were normal Y-class 3x3. You get normal LX service (i.e. a cheese "Laugenbrötli) and drinks (so not BOB as BMI), including a mini-Toblerone. So it's very much LX.
You end up at LHR T1 at the far end of the terminal (gates 25-27), so it's a long walk back to the terminal and the whole of T1 looks rather tatty.
My return flight was a regular LX flight. About 7-8 minutes to get through security.
The pleasant surprise was the BMI lounge. It's a bit difficult to find and a bit out of the way above Gate 5, but it is worth it if you have time to spare. In my opinion it's way more pleasant than the average FTL lounge. I was warmly welcomed by the lounge dragon (totally wrong description for this one), who having discovered I hadn't previously been there, explained where everything was. Fine view of runway 27R from the lounge. Pleasant seats with lots of privacy. Decent food and drink offerings.
Gate again at 25, a good 10 minutes walk away in a section of the terminal without ceiling panels, and without heating. Everyone ended up putting their coats on it was so cold. LX flight was normal.