So I hope that the contest organization will inform the FIS (sorry, Basic) providers in the area (not just the LARS providers as suggested in the OP, but London Info as well) of the actual tasks set, the actual starting time and the estimated maximum altitude of the gliders, so that they can inform others.
The Competition organisation has a dedicated airspace liaison official, who informs LARS providers, London info (and any other appropriate interested parties, such a Salisbury Military, Brize, and so on) of the actual tasks as soon as they are set, and likely launch time based on forecast met.
Start times are at pilots discretion once all gliders have been launched. The task groups are widely spread along the task route, although as has been pointed out, in difficult soaring conditions there is a tendancy for faster pilots (who typically start later) to catch up but then 'drag' slower pilots with them in a 'gaggle'. A circling group of gliders is easier to spot from a distance than a single one flying in a straight line.
How much of the task iinformation is passed on by a Basic Service Provider is not under the control of the organisation. Contrary to some impressions, modern gliders are very visible on primary radar, having carbon-fibre spar booms which have a high radar cross-section. The NATS radar traces from the day of the unfortunate incident in the Abingdon area recently show this.