How do I bathroom / kitchen / extension? etc.

If you know exactly what you want, then in my opinion you don't need an architect. The question will be whether the wall you want to knock down is load bearing, if it is you will need a structural steel put into place, so will need a structural engineer to guide on size etc.

I personally would try and get a couple of builders out, good ones will tell you what you need and and will also have structural engineers they have worked with before.

Seconding what @Sam_w says. A half-decent builder will advise you on the process anyway.
My experience:

1. Choose a builder
   
2. Get structural engineer to do some calculations
   
3. Share those with local council building control
   
4. Crack on with knocking down wall and inserting steel
   
5. Building control come and say 'Yes very nice'
   
6. Carry on with building work
   

Please use terracotta tiles if a 60's terrace (I really hope they are not the tiles you talk of ripping up).

The benefit of an architect is they will consider every little detail (if they are good).
I recently had a bit of a nightmare with my bathroom floor as I have been planning it all myself and had no idea how to build up a sub floor resulting in a 50mm threshold change when going tie/out of bathroom.

Luckily the architects I used for the extension still tolerate me and were able to spec up the floor to reduce that threshold, whereas the floor supplier was giving out a bit of a blanket recommendation.

You defo don't need an architect, but they will bring a finer level of detail and consideration than a builder alone (as that's their job) and hopefully add a "delight" factor to the space, whereas a builder will likely only add square footage.

You could of course just crowd source every decision like I have done.

As King of the Castle said, you'll need a structural engineer regardless, so I'd be inclined to do self design > structural engineer > good builder.

What you are after isn't massive so shouldn't cost loads, I would imagine sub 10k but no real idea.

I have a 60s house and did a ‘modest’kitchen about that size - white magnet units, cheap tiles, solid wood worktop , forbo floor - cost about 14k including Bosch appliances and a 60cm smeg gas range. Including striping out the old and replastering ceiling and screeding the floor.
So 10-15 for a kitchen , 1-2k for the opening and plastering 1-2k for new floor
Plus contingency / extras - £12-19k ?

Costs for me for a new kitchen and raising an exiting steel in a 60s build as follows:

The room and kitchen are bigger , and I'm in the North West , so not quite like for like with yours:

1. remove existing steel, add new thinner steel to sit inside ceiling cavity, brace 1st floor joists , make good. 2k
   
2. Pronorm kitchen, silstone quartz worktops, Neff built in appliances 16.5k fitted
   I looked at Ikea or DIY Kitchens carcasses with custom fronts, bust the custom fronts I found were not cheap at all, all in it pushed the cost close to the above , with an increase in coordination and project management overheads
   
   

There will be some additional plumbing and electrical work, but this has been included in a wider scope of works, so I don't have standalone costs for just the kitchen.

flooring- if tiles are cold it's likely wood will also be cold (though not as cold). You may benefit from some kind of sub floor insulation if the tiles are laid straight over concrete. Im also having to do this.
Reasonable engineered wood seems to start at ~£30 per sqm, cheaper stuff has a very thin layer of veneer and much less solid looking lower layers, and i suspect is a false economy. Labour / fitting costs will depend on the amount of prep needed

Doubtlessly, i could get the kitchen and appliances done cheaper, but all depends what you want