I think BA are quoting the time with waiting at baggage reclaim and so on. It could take up to 75 minutes, there will be a bus from terminal to terminal again you have to wait for this.

When i went to america we went from manchester to heathrow and went to terminal 1 and we neaded virgin atlantic at terminal 3 do we just walked it and it took us only half an hour

If you are travelling with the same airline, or one in the same cartel, your luggage can probably be ticketed through to your final destination, so you'll never see it at Heathrow. (Mind you ... you might prefer to re-check it in as there have been so many bags that have missed their connection).

I also had a very short connecting interval from T1 to T4, and was surprised how quick the transfer was (using the internal shuttle buses) - even though the 'paper' time estimate was also 75 mins.

Not travelling with the same airline.
Thanks to all who replied ...............I am now a bit clearer as to what time I should book my flight from Glasgow in order to leave enough time to get from terminal 1-3 plus the 2 hours or so needed before next check in.
Pat