Any guesses on MAD right now are just that, guesses, and probably not particularly well-informed ones. Here's what needs to happen:

1. Airbus advises BA of A350 delivery date based on progress down the line plus flight tests. There's supposed to be a 30-day heads-up on this, BA has been saying July for the delivery, so we won't likely even have a delivery date until sometime in June.

2. BA picks up the keys to the A350, does a TLS-LHR ferry flight - again, sometime in July but which end of the month is anybody's guess.

3. The A350 has to go through an 'entry into service' program, this isn't my area of expertise but I believe this is a few weeks. I know that it will encompass some additional cabin work because while the Club Suites are being line-fitted at TLS, there's 'finishing work' to be done by BA on arrival.

Taking that into account, I'd not expect MAD to pop up on the schedule until mid- or even late-August. If you want to get a jump on this, you could set up an aircraft change alert in ExpertFlyer for the likely flight. However, don't count on getting into Club World on the very first MAD flight as BA tells me their current thinking is to book out the entire CW cabin for media etc (likely also some BA staff who have won contests etc).