Last year (bid 2018 for this year’s training) if you joined on the A320 with the required hours you could get a command after the minimum one year in the company. Long haul engagement freezes were waived for about 10 or so pilots bidding for Gatwick commands. This year the most junior Heathrow 320 command in 2020 is about 3 and a half years in the company at the point of the bid results but will be unfrozen for command. The pre command freeze is 4 years so effectively lops a year off your engagement freeze if bidding for an available command.
Current BA policy seems to be that seniority doesn’t apply for commands during the engagement freeze - if you are on the 320 and meet the requirements you will get a right to left ahead of senior but non-rated bidders. This is a consequence of the fact the agreement never anticipated such junior mainline commands.
Bottom line, although relatively junior commands are available, bank on 4-5 years if you want LGW and up to double that for LHR. Don’t come here if you have your heart set on a quick upgrade as disappointment is likely to be coming your way.