The only other airlines flying Aus (SYD specifically) to LHR are BA and Virgin Atlantic with one flight a day each. Qantas has double that with A380 capacity.
Now yes other airlines (EK/EY/SQ/TG/MH etc etc etc) may well be able to offer through routing several times a day, but only because they are flying through their home HUB ports where they can combine end to end passengers with passengers from dozens of other flights thus making it viable.
News in today Virgin Atlantic are pulling out of Oz. So that makes just QF and BA left as true end to end operators.