If your BA flight is not the last flight of the day, I'd call now to have it changed (assuming it's one of the tickets that allows free same-day change) to a later flight, but I suspect you might well be on the last flight of the day since you mention 'evening flight'?

I cannot recall the precise details of transfer procedure (esp. domestic to international) as OTP as I haven't connected there despite having used it rather often, but with 45 min. from landing to departure (with gate closing earlier than 45 min after landing of your previous flight) - I wouldn't like your chances. Even the exit immigration can have a bit of a queue at OTP.