Will the Thai Embassy in London accept a bus ticket as proof of onward travel for my 60+30 visa application?

You could ask the embassy. Both aspects (bus vs flight, and 60 vs 90 days) are within a grey area in which both interpretations could be valid.

It is generally assumed that only the original 60 days qualify, but as many embassies also understand how unimaginably weird it really is to require that while at the same time offering travelers the opportunity to extend by 30 days, many do actually accept onward travel within 90 days instead.

I think the bus ticket is less likely to be acceptable by an Embassy, but some airlines do accept it in case you'd use visa exemption. It's hard to deduce what immigrations requires upon entry, because they never check. The rule is that the officer "could" accept it. So he/she could also not. Some airlines argue that if it's addressed this way it must be for a reason and that it hence should be ok. Others apply the stricter interpretation. But if I'm correct, most embassies accept only air travel.

So ask your embassy to know for sure.

Used a bus ticket myself from nong khai to Vientiane £12 was my proof of onward travel think it was 12go I used just Google bus nong khai Vientiane comes up it works as proof

I had one not long ago and I had booked a 90 day flight and the check in desk were perfectly aware that the visa could be extended by 30 days. I don't believe they will accept bus tickets but there are plenty of cheap flights in and out to adjacent Counties.