Removal of back door?

Yes in most cases you can, if you are not making structural changes to load-bearing walls if it might you will need a builder to sign off on it. Also if its not a listed building and you still have a safe exit from the building in which case you do through the Patio doors. I would get this looked at by a builder and they can advise best once having a look on the structure and how best to do this the safest way.

Windows & Door specialists typically deal with these jobs.
Needing building permission on the other hand I'm unsure off. It's always best to contact your local council and just ask them the question, Rather safe than sorry!

Yes, you can remove your back door without planning permission. Hire a builder or joiner to do the work.

In most cases you won’t need planning permission to do this unless your property falls under certain criteria such as listed buildings. If you are unsure it doesn’t hurt to ask your local council to reassure you they will help with any enquiries you may have.

Removal of back door?

Yes, you can usually remove a back door without planning permission, especially if you're not altering the structure significantly and still have another exit (like your patio doors). Since it’s part of your kitchen and you're looking to reclaim space, this is a common job.

That said, you may need to make sure the wall is properly insulated and finished where the door used to be, and it still needs to meet Building Regulations for things like ventilation and fire safety.

A builder is the right person to handle this—they can remove the door, block up the opening, and make sure it’s all done properly. A joiner might get involved if there’s any custom carpentry or internal finishing needed.

Removing a back door is usually straight forward but there are a few things to consider such as building regulations, since you have another patio door the fire exit situation shouldn't be an issue but you will still have to make sure that the kitchen still meets ventilation and escape route requirements, you can check this with your local building control.

You also normally do not need a planning permission to remove back doors unless you stay in a listed building or conservation area.

Also checking there is proper support if the door is removed in case it is located in a low bearing wall, check this before removing.

if internal alterations are required then plastering and decorating will be needed as well as adjusting skirting boards.

Yes you can block a doorway. Probably better to ask a bricklayer/ general builder