+1 to this.

And be very careful because if it isn't properly processed and then IRROPS messes with your flight later, you could very well have the BXP kicked out into the void.

I recently had a situation where I bought J on BOS-LAX, used a BXP1 to upgrade to F. That was on a Saturday, and overnight that night there was a schedule change. So the next day I had been rebooked in regular first class on something like BOS-ORD-LAX (because AA torpedoed BOS-LAX frequencies). No problem, I thought, I'd just call and switch to BOS-JFK-LAX -- but they refused because the BXP1 upgrade hadn't actually been reticketed. Even though I had an email showing "confirmed" neither the EXP desk nor the Business Extra/Meeting Services people would budge. The BXP1 was automatically reinstated as unused in my account (before I even called), as though it had never been redeemed. Super frustrating, eventually availability popped up for a mileage upgrade so I just did that instead.

Used to be my view that these tickets didn't actually get reticketed, but after that experience, there is at least some measure of reticketing that has to happen before the upgrade is truly confirmed. I'd like to think that with LHR it will be like a SWU, there will be a proper email reticketing confirmation. Better get that ASAP.