To many AFFers, such a trip (SYD LHR rtn) should be best booked directly with the airlines where you have no agent's fees and get to deal with seat selection directly (pay $$ and get an exit row seat etc...).
Complicated itinerary (say SYD to Kilimanjaro JRO) probably requires some help for the uninitiated.
But I appreciate people has different skill set and they would act differently.
Not just specifically addressing
legroom's comment but others who seem to favour dealing direct with the airlines.
I'm a pretty experienced international flyer but ALWAYS use a travel agent for international. Its a moderate sized, Melbourne based one, who mainly deals with corporates (but anyone can use them); 50% of my travel is for business, 50% private, always (or nearly always

in J.
Don't know how the likes of Flight Centre etc work, but the only time I'm not able to deal with the airline direct about my booking is prior to the first flight. After than, I do whatever I choose, direct, except if there is a change in itinerary etc where there might be payment implications for the client.
TAs take their fees (or
should take their fees) from the airline, built into the same ticket price. Same with hotels. I have priced privately on line, and then got the same price from the TA. Sometimes the price is cheaper because they get the AMEX discount and pass it on to me (or some of it!). When there has been a lot of mucking around on an itinerary, eg where I change my mind or the TA has to hunt down availability, I offer to pay some time based fee for this, but they have never accepted.
TAs have access to deals and options that you simply can't get on line. Finnair's great prices to Europe, including a free intra Europe return trip is a case in point.
And if the itinerary goes pear shaped (such as if you are in Morocco and Lufthansa cancels your flight to FRA tomorrow), then the TA sorts out the mess 24/7, rather than you having to make many calls to some airline call centre several countries away hoping, like 200 other people on the flight, to get fixed up.
I wouldn't touch the likes of Flight Centre or the others like it, but I love my Travel Agent! I'm happy to give
becinbloom (or anyone) the name of my TA if you'd like to PM me.