Officially the "Minimum Connection Time" for United Domestic -> United International at SFO is 45 minutes, and as you're above this it is still what is considered a "legal" connection.
However all this really means is that if you miss your connection the airline is responsible for finding you a seat on a later SFO-LHR flight. It doesn't mean that you'll make your connection, nor does it mean that the airline will cover any additional expenses such as hotels during the delay, etc (They might, but it depends on the cause of the delay).
As both of your flights are on United you do NOT need to re-clear security at SFO, nor do you need to collect your bags as they will be checked all the way through from LAS to LHR.
Presuming that your LAS-SFO flight has a flight number below 2000 then you inbound flight will arrive in Terminal 3, and your outbound flight will leave from the International Terminal G. These two terminals are connected by an air-side walkway, and depending on which gates you land at and depart from it will be in the 10-15 minute range to walk for an average person.
If your inbound flight is a flight number above 6000 then it will arrive in Terminal 1. There is an air-side bus from Terminal 1 to Terminal G, but depending on how busy the bus is it could take up to 20+ minutes to get between the two terminals and to your gate.
Your outbound flight will start boarding around around 45 minutes before departure, and technically you need to be at the gate 30 minutes before departure time, although realistically 10-15 mins is normally the minimum - any later and you risk losing your seat.
So basically, if everything goes right and your inbound flight is on time then you'll have no trouble making your connection. You probably won't have enough spare time for anything other than a very quick bathroom break, but it's doable.
However if your inbound flight is delayed even a little - which is not uncommon on LAS-SFO flights, especially if it's a morning flight - then you will likely miss your connecting flight. Depending on the time of year there are only 1 or 2 direct SFO-LHR flights per day on United, so the impact of missing your flight is very high - most likely an up to 24 hour delay!
If it were me, I would be changing to an earlier LAS-SFO flight. United is normally very happy to do this when there has been a schedule change. In this case as the ticket was booked via an agency and as a codeshare they may not be willing to assist, but I would start with them presuming you can conveniently call them. Otherwise talk to EBookers and ask them to arrange the change. In cases like this it's normally best to know exactly what you want before calling (eg, you want flight UAxxx from LAS-SFO rather than your existing UAyyyy).
