Also, London is big. Where you start from in London is very important.
The following route sneaks through the Chilterns with minimal climbing. Assuming you start in Hyde Park, which is as good a guess as any: Hyde Park, Ealing, Southall, Iver, loop round the N of Slough to avoid the centre, Maidenhead, Warren Row (Velolife Cafe), Henley-on-Thames, Christmas Common, Brookhampton, Oxford.
That's just less than 700m of climbing in 106km. You'll struggle to get significantly less climbing than that. As 100k rides go, it's on the flat side.
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And at 60km you have the opportunity to have some amazing cake at the Velolife Cafe near Warren Row to fortify yourself for the only real climb of the day up to Christmas Common. The Chilterns are in the way. You can't avoid them, and this is seems like the easiest way through.
I've not ridden that route myself (other than some nice bits around Christmas Common and other nice bits near Warren Row) so no guarantees. You will need to do your own research, this is just a suggestion and could have some poor choices.
The first 30km (up to the M25 just before Iver) will be horrible - it's a case of studying the roads, asking on here for local knowledge, and making a bearable London exit, avoiding the biggest of main roads. "Bearable" can be counted as a success. But at least it's flat.