Just for your info, unlike US airports the various airlines do not have any "ownership" of the gates in T3 but are just allocated to any gate from day to day, which are shared with all the other operators. Looking at the history over the last month, UA959 has departed from a whole range of gates, differing each day.

Note that operational information may refer to "Gate 313" (as you do), but within Terminal 3 this gate is just signed as Gate 13. each terminal is numbered like this, so over in Terminal 1 there is also a Gate signed as 13, which is accordingly known operationally as number 113.

The signing of the gates is very good in the terminal. Also United operate those battery-powered little vehicles between the RCC and their various gates if your folks wish to take one. It can be quite a long way and the moving walkways are renowned at Heathrow for being broken down.

Terminal 3 gates are numbered between 8 and 42, plus there are some "gates" which are in reality just doors to shuttle buses used when the aircraft is parked on a remote stand. Quite a few United flights are parked on the 500 stands, over where Terminal 5 in being constructed. It takes the buses about 5 minutes to get you over there, and you board the aircraft by open steps.