Don't know about the C-checks cycle for the 318; I sure know that, with age, the efficiency of a plane declines. Back a few years ago there was a working group to look at their (when there still were 2) efficiency and range. If I were to hazard a guess, I think the fate of the 318 is, in order of likelihood to happen:

1) do the check and continue functioning for a bit longer
2) dismiss it, close (aka "suspend" in BA parlance) the route and issue a press release to say that customer can enjoy Club Suite and the 6 gazillion pound investment, new lounge and blah blah blah
3) buy a new fleet (the A220) for exactly one plane, perhaps more once LCY gets larger and BA renegotiates its deal with BALPA.