A connection of that size is indeed a gamble (but one that I would take). My experience is that if you fail conformance you will be booked on to the next available flight with seats. At that stage they don't distinguish between flights with points availability - they just look for a seat. If the next flight is already full they will probably offer to put you on standby with a confirmed seat on the next fully available flight. There might be a discussion about voluntary downgrades but that rarely happens. CE to TXL is not that popular, so I doubt there would be a problem.

Failing conformance means when they scan your boarding card at LHR just before the security check you will need to be there 35 mins before the TXL flight - which could also be a bit late of course. Printing your own boarding card may give you 5 vital extra minutes over not having one.

However a part of me would imagine that missing that flight would not exactly be the end of the world since you would have access to the Concorde Room by dint of your arriving F flight. Indeed some people would relish 12 hours there! Which is why I would be happy with that 1 hour connection, there's always another bus.....