Glad it worked out, but it is risky to plan arriving ahead of your longhaul flight on the day of departure.

It is not unheard of for London flights to be scrapped at short notice, depending on loads. You could be very unlucky and find yourself on a new connection with little (or no) time available to make such a connection.

For peace of mind - it's best to get to the origin of the "main" ticket the night before, if you cannot book all travel on a single ticket.