I have a pitched roof that needs a higher ceiling beneath

Hi do you have any photos you could add, so we could get a better understanding. Thanks

You are describing a duo-pitched roof with a horizontal ceiling tie (timber) which is sat on timber wall plates. The ceiling tie prevents lateral thrust from the weight of the roof, tending to push out the external walls. You cannot just simply remove this ceiling tie as it provides structural stability to the roof and wall. However, it should be possible to raise this ceiling tie, up 1/3 of the height between the underside of the ridge board and top of wall plate. This would then be called a 'collar roof'. Note that the joints would need to be bolted together with tooth-plate connectors and washers. This is a structural alteration, and you would need Building Regulation approval before starting work.

The horizontal timbers are part of the structure of the roof. These cannot be removed .

No you can not remove them the timber ties in the rafters sitting on the wall plate remove these & the roof will collapse