As long as the -800 is available they will never take any other airframe into the operation:
The whole FR-system depends on flexibility. If one aircraft at base A needs to go into maintenance, it is simply changed out at a 25min turnaround at base B. The same thing happens if an aircraft goes tech - spare is shuffled in within a couple of hours.
In short term planning a -700 might be an option - but what happens, when economy in Europe starts to go up again? The just-arriving -700 will be too small and they exactly have the problem as described in line 3.
FA10