Mr Punter

I am not sure I quite get you but if it is not level you will need to pack it up completely level. If you pack under the DPC you will later be able to gun in some mortar between the packers. Do this from both sides to fill any voids. If it is out of line you will either need to do a redesign on the frame or you may be able to overhang the soleplate by a small amount.

You could have 15mm plasterboard, 25mm service cavity, vcl, 140mm timber frame filled with 130mm pir (90 + 40), 10mm sheathing, 50mm cavity and either render board system (15mm) or brickwork (100mm). With the render board system you may be able to reduce the 50mm cavity to 25mm, so for render board you will have 230-255mm and with brick it is 340mm. I think this gives a u-value of about .19 but sometimes it depends who does the calcs and what flavour pir. I think you can claim the service cavity as a low e gap.

Small children getting their heads between the wires is the risk.

Most of the plastic 450 chambers have inlets at 90 deg to the main channel. The outlet has to be from the main channel but you can put a 45 deg bend on it and still rod OK. You can also put a 45 on the inlets, so lots of flexibility.

I have RK doors x 8 on a development. Cost a fortune but look OK.

The new IC will be fine in terms of inlet numbers and angles. It is important to check invert levels but the connections from the utils and kitchen will be above the main sewer pipe and the connection from the new IC to the sewer will either be at the same level or on top if it is a saddle onto a larger pipe.

Without seeing the rest of the front elevation it is difficult to suggest. A timber finish can often bridge between traditional and modern. The big vertical handle seems to be what planners consider "contemporary" but they can be a bit clueless. Also consider if you are having a letterplate and if so where. Do you want an external lever handle? Do you need natural light? Is there to be a bell push / entryphone? As a general point it is very good to have a canopy to protect from rain.

https://www.jameshardie.co.uk/product/accessories-and-tools-cladding/ the ventilation stuff

I have used the perforated metal stuff from James Hardie. They do a bottom profile which gives a kick to the first row of cladding or slate and a l shaped top profile.

We have sliding door wardrobes and the end doors have soft close which fits to the top of the door. I can see that @nod is a dab hand with the metal stud!

Petrol masonry saw?

I think @Dreadnaught is about right with his thoughts. The main things affecting £/m2 sale prices and desirability are 1. Location 2. Kerb appeal You cannot do anything about the location if you already have the plot but the front elevation is what people will see on Rightmove so make sure it looks right. For the space you have downstairs: open plan kitchen living dining (with divider if pos), downstairs WC / utility (a utility with a WC), study / desk area. upstairs: 3 beds, 1 bathroom (bath with shower screen), en suite shower (large shower). Wall space for storage in each room. Plan the layout very carefully so you don't have wasted space. Floor coverings: Hard flooring downstairs - same throughout looks sleek. Carpets to stairs and bedrooms. Luxury vinyl tile to bathrooms. Gas CH. Don't bother with UFCH unless you have tiled ground floor. Combi boiler. No home automation.

Pedestrian or vehicle?

Once you have stripped back to the membrane I think you will probably not get too much water ingress. Then get it PROPERLY designed and detailed - including detail sections through each junction. Then post the design here for comment. Then build it to the design.

Well played! It is only a row of chipboard boxes.

I don't know of any you can drive on. I have used some plastic pedestals but they are only for pedestrian traffic. I imagine you would need a fairly hefty galv steel support system to drive over and would be £££

I suggest a liquid product so that you can easily detail around and onto the upstands.

Good luck! I saw a firm called Plykea who do ply and Formica doors, plinths and panels for Ikea Metod units. You just send them plans and they work out what you need.

Off topic but a nice job on the paving. What make is it?

A BBA approved render carrier board (such as Bluclad) fixed to vertical timber battens. You do not need an approved installer. You can then choose the render and colour at your leisure.

I have never seen these before so googled https://www.strongtie.co.uk/products/detail/slotted-z-clip/79 They are for noggins. I was expecting truss clips. Ask the designer if you need anything else (holding down straps etc).

BTW is the steel still structural or was it just temporary? It looks badly corroded.

If you have membrane on the ceiling it will need to have a fall to the walls or water will just collect and pond up there. You could just pitch a tent!

I have heard it called Mum and Dad but only blockwork.

The house looks very handsome. Compared to the flint work the rest of the job will feel easy - unless you plan on making your own clay tiles.