The gate info in departures is typically not available to the passengers until 30 minutes before boarding because the airport / airline does not want departing passengers at the gate before then.
Often because with (say) easyJet a flight is only on the ground "turning round" for 45 minutes so they (or somebody else) are probabaly using the gate for a different flight up to about 45 minutes before a particular flight departs and they don't want confusion, overcrowding or passengers loitering in the corridors.
Additionally, the UK mostly has "closed" gaterooms which require handling agent staff to check the passengers boarding passes and passports before letting them into the gateroom (in the US they have open gaterooms and the checks are done just as passengers board). As those are the same staff who operated the check-in, they won't be at the gateroom until check-in has closed, which is almost when the passengers are scheduled to board.
A final reason is because there are often last minute gate changes as an aircraft arrives, because for a whole host of reasons a departing aircraft may not vacate a stand when it was planned to, so stand allocation can be a very dynamic business.