Stud wall knocking down and extending

Hi everyone,

you guys know your stuff so please help me. I’m ok at diy and I have built a standard stud wall before in a previous property . I have explained the situation and the pics below

situation:

floor plan pic- red is stud wall b being removed
Yellow is stud wall a
Blue- is stud wall extension being built

pic with pink door- circled is the wall being removed

I have a very small walk in close in the corner of my room (on the flat ceiling side) that I want to reconfigure by knocking down one stud wall and extending another.I live in a. Victorian attic conversion flat and the roof is above my Bedroom. It has a half sloped ceiling and half flat.

I want to extend stud wall a (currently measure 80cm and I want to extend to 242cm) and I want to knock down stud wall b (158cm) . Stud wall b is hollow plasterboard with one stud in it.One side of it is connected to a solid wall leading for the neighbouring property. It doesn’t seem to be load bearing .This will make the close 3 walls (2 solid and 1 stud) rather that 4 wall it is currently.
I know which way the joists run and they run directly parallel with the stud wall extension I want to build but perpendicular to the wall I want to knock down. One side of it is connected to a solid wall leading for the neighbouring property. It doesn’t seem to be load bearing . I have a few questions :

Advice needed on:
- is there anyway to build the extension to stud wall a without drilling into the ceiling joist? I’ve seen that the Americans use something call sill seal gasket to put the head plate and foot plate on when building temp stud walls but I can’t find it in the Uk when I type that in?

- what is polyethylene foam sill seal gasket called in the Uk?

-what is the thinnest timber I can build the wall with. The wardrobe will be going in front of it so it’s more to hide the back of the wardrobe and will have no weight bearing on it?

Best way to connect the stud wall extension to the existing stud wall?
Any advice welcome