In the good old days, the BA system fed a link to Ceefax (or Oracle in those days) every 5 minutes. Little better than a piece of wet string!! but it seemed to do its job most of the time.
The BA system took its info from the BAA system which in turn came from ATC, adding 10 minutes to the 'finals' time to provide a 'landed' time.
This arrangement obviously had no compensatory factor for go-arounds etc. That was how it worked at LHR and I imagine similar at LGW.
One would like to think it has been updated somewhat these days as I am merely providing an historical description.