I guess you are in dispute with your builder on this? The victorians used, i beleive, clay for their pipes and they probably will break as soon as they come out of the ground and have an awkward imperial measurement that will require replacing with plastic encased with stones and in lintels. I’d say it is an additional £1-2k if it wasn’t in the quote / spec / estimate you originally got.
Bigger issue is whether you have rights to alter it in any way.
If it is a shared drain on your property then you do not own the drain anymore. Either alone or with other households. Since 2010 ownership and responsibility for drains that are on your property and which serve more than one property are the responsibility of your sewerage company (Thames)
You neee a build over agreement from them before touching it. They want to make sure that the integrity of the system will not be impacted. As this could cause problems elsewhere. You need to provide drawings and details on the design.
You can risk moving it without asking, but it will cause you problems if they find out you did the works without permission.
Under no circumstances will you be allowed to put a manhole cover in your kitchen as it makes access a lot more difficult, and flooding more expensive for the water company to deal with. Even the bolt down sealed types are not allowed. And you do not want any bad smells, which will come.
They also need an inspection cover at least every three households so they can easily unblock it. Have a look on the Thames water website for all the things they look for (developer services ?).