Spotted...

Going East I always took Kensington high street: no hill.
Going west I took Holland Park Ave and tried to break my speed record going down it. I took the computer off in the end as it was getting dangerous :-)

You can avoid Marble Arch and Bayswater road by going North of Oxford St and up Craven Hill, which is shorter and steeper.

I avoid all of that anyway, by cutting through hyde park, across park lane and using a bit of the tweed run route I remembered to get to picadilly and Traf Sq then go down the Strand. Niall..that is not a hill.
Shep Bush Roundabout isn't actually that bad on it's second day. I still hate HPA though..but i'll find another way.