I've taken BA 143 a few times, including this year in March and the immigration process has been pretty quick (I guess maybe even quicker for you as a national, but you will know better) and baggage ok.
If memory serves me correctly the transfer is just a few minutes walk between terminals.
I think this is more than ample. My guess is the issue will more be not getting too tired or bored waiting for the next flight through the wee hours.