MCT is 90 minutes. This is realistic, you will likely take about 60 minutes (less if you move fast and the buses are with you) to arrive airside at T5. Bear in mind there is a security check at T5 with the usual liquid restrictions.
You will likely have a short time in the T5 lounge. Note that the aircraft door for a BA flight will close at T-20 minutes, do not be late. If you are in the GF lounge, gates in the 14-18 sort of range are within 5 minutes walk of the lounge, 1-10 are more like 10 minutes. Ask the lounge entrance staff for the expected gate of your lounge.
Originally Posted by
LchChester
I would say 2 hours in order to be safe as you will need to get on Heathrow Express or the Tube to get to T5
Not necessarily. If your bags are checked through, DO NOT go landside for this, instead follow signs at T3 for "Flight connections" and take the airside bus to T5. CX251 may well arrive early, so you'll have a couple of hours at least in the T5 GF this way.
If you have to get your bags because of the unhelpful new OW policy causing your bags not to be checked through, then you do have to use the HEX (or Tube with contactless or Oyster card), and you will likely take about 75 minutes to arrive airside at T5.
Keep your CX boarding pass to show the lounge staff to assure entry to GF rather than GC. If you want a shower, ask at the desk to the right as you arrive on the GF entrance level before you go in the lounge, you may have to wait a little while.