I've done this route several times in the past, including on my QXC. If navigating visually without GPS I usually route Henley-on-Thames to Sandhurst then via Farnborough's overhead. That route keeps clear of the Blackbushe ATZ and the Heathrow CTR.
Both Henley and Sandhurst are very obvious landmarks from the air; the former due to the change in direction of the river, and the latter due to the Royal Military Academy buildings, parade grounds and the lake. The route between them is also fairly obvious due to the large junction on the M4 and then the railway line running south-east from Wokingham.
Farnborough have never had a problem with me transitting their overhead (though as previous replies have said, they sometimes ask you to route to the east or west rather than directly overhead if there is traffic on an instrument approach).