No, it is not illegal to open a sewer cover in the UK if the sewer is on your property.
Is This Something That I Need To Worry About?
The manhole cover probably leads to a public sewer that serves several other properties nearby.
Due to the risk of causing harm to the sewer, all construction activities close to a public sewage system are strictly restricted. A manhole cover cannot have a structure built over it.
Additionally, you must obtain the local water authority’s approval before erecting anything within three meters of a public sewer. To guarantee that you continue to comply with their rules, they may ask you to sign a “build-over agreement.”
ALSO SEE: Can I Run Rainwater Into Sewer in the UK?
Prior to beginning any work, it would be necessary to consult your local authority’s building control department.
It might be wise to think about changing your conservatory’s plans so that it would be at least three meters away from the sewage or manhole cover.
As an alternative, you could request that the neighborhood water company remove the manhole cover and redirect the sewer.
This, however, would be at your expense and most likely be quite costly.