It is possible to travel direct to Zhytomyr, from London, thanks to a Ukraine-based operator.
The service leaves london Greeen-line, promptly, at 20:00
(I emphasise 'prompt', because there is a UK-based, 'cowboy' operator, Eastern European Travel, offering identical itinerary, which they do not even attempt to abide by, losing time by leaving London late & making no attempt thereafter to maintain their timetable, so arriving in Kiev (final destination of coach), 10 hours later than schedule...)
If you chose the Ukrainian operator, you'll receive professional treatment & be in Zhytomyr at 09:00-11:00 (dependent on whether passengers are booked for L'vov, which means a less direct route.)
I would not care to do this distance on a full coach, but travelling out of season means a seat to yourself, so you can sleep when you please, though only the German autobhn sections are boring, the scenery of the rest of the route through 5 countries, is the main incentive to take the slower option. As is the duty-free on the cross-channel ferry, though the highlight of the entire journey is stopping halfway between Rivne & Zhytomyr, at the Talee-Yana restaurant, where you can eat traditional Ukrainian food at its best, beside a wood fire in a grate...