I expect your kitchen tap is a flashy stylish Italian design with ceramic discs, perhaps a joystick, and tiny little tails the size of pencils. If you get a UK-made tap designed for our lower pressures, it will have larger internal waterways and larger connections, and will give better flow. Single-hole taps are more cramped so have less room for the water to get through. The greatest flow comes from a UK Pillar or Bib bath-tap which has large connections, large waterways, and large flow.
Flexible tap connector hoses, and ball-o-fix service valves, also contrict water flow, unless you make a point of buying large-bore ones.
I expected it to be difficult for you to lay a new pipe, but look out for any plumbing work done for the downstairs flat and see if you can get yours done at the same time. It would be interesting to know what your incoming water main is made of, and if you have your own meter or stop-cock in the pavement outside.
Even if you decide you want to put a pump on the incoming main, it is not allowed.
If you can find a more competent plumber, why not ask him to propose, and actually do the work? better than having a known incompetent plumber do it. Any plumber who tells you to put a pump on the mains supply is incompetent.
The bathroom/kitchen contention is because you have a limited amount of water coming into your home. If you open one tap, all of that water can come through that one tap. If you open two or more taps, or flush a WC, or your washing machine is taking in water, then that limited amount of water is shared out between all the taps. If you have no cold water storage tank, then the mains supply has to be shared between all the hot taps and all the cold taps. If you have 12lpm coming into your house from the main, you can have 12lpm coming out of the hot bath tap, or 6lpm each coming out of the hot and the cold bath taps, or 3lpm each coming out of the hot and cold bath taps and the hot and cold sink taps, or 2lpm each coming out of the hot and cold bath taps, the hot and cold sink taps, and the hot and cold basin taps (actual amounts at each tap will vary depending on pipes and taps, but the total amount will not vary).
This is a very common problem for people who convert to a combi with no storage, and do not have a large incoming water pipe. A combi with poor incoming water flow is OK for a single person with one bathroom. It is increasingly unsuitable for homes with more than one bathroonm and more than one occupant.