Originally Posted by flarmip

A B2B at JER is as risk-free as it gets. 95% of the time you get off, turn left along the corridor and hang out in the lounge that's 10m down the corridor for 5-10 mins until boarding is called from the gate a further 20m down the corridor. 5% of the case you may be deposited at a different gate that requires you to go landside and through security for your return flight, but even that isn't an issue. There is no conformance like at LHR, so you can get through security really "late" without issue. JER is a very small and efficient airport so even going landside barely adds a few hundred metres to the distance to walk.

If the plane doesn't make it to JER (diversions back to LHR due to bad weather aren't unknown) then the return flight to LHR also won't be going anywhere - so BA will be rebooking you on your TP run regardless. In one such case, during Storm Ciarán last year, I was able to get the JER-LHR flight removed from the booking to avoid having to rebook onto other flights for my "actual" journey. I even successfully claimed ORC for the JER-LHR TPs I should have earned.

You definitely want to go for the BA1348, however - if you go for the 1346 and it's cancelled whilst the 1348 still runs, there is a risk that you could be unable to get on the 1348 if it's fully booked. That really could scupper your plans!