I agree with the above. 55 min can be very tight. T3 security is a lottery depends what time of the day you need to go through especially during holiday season. It can be easily 20-25min on its own. Also if you arrive at the morning usually a flight leaves around 10am (if that your intended flight to LHR) the following one is at around 2pm so long wait. I would only do it if the price difference really high. Also don't forget the HEL-MAN flight earns normal tier point (ie 80 on CE and not 160) so with connection you would earn more or less the same as a direct flight. Avios on the other hand would earn more on this connecting flight. (few hundred depends on who ticketed BA or AY and what fare bucket, etc)