I am actually astounded at this figure of 37% for transfer traffic
Technically it isn't 37% of passengers using LHR - that percentage is derived by counting every transit passenger twice, once on arrival and again on departure.
If you're confused by that, you're in good company - earlier this year the GLA's Transport Committee asserted that "There are disputes about the extent to which Heathrow is operating as a hub airport. It is not clear how many of Heathrow's passengers are transferring with estimates ranging from 22 to 36 per cent".
http://www.london.gov.uk/sites/defau...%202013%29.pdf (Page 5)
It turned out that there is no "dispute" - they simply didn't understand that there are two different ways of calculating the proportion of transfer passengers.