It is great that you are going to be very careful but even if you are very careful, things can go wrong.
I will try and find the link, but I remember reading about a poor woman who did everything right before contracted with a builder (she ensured he had adquate insurance etc) but when her house collapsed, the builder's insurance refused to pay out because they said that the builder had not followed correct procedures.
I will try and find the link, but here is a similar story:
www.thesun.co.uk/money/14979289/devastated-family-left-250k-bill-home-collapses-extension-work/
Remember to tell your insurance company before you have ANY work done (even if you are just having the kitchen re-done) as otherwise you will not be covered if anything goes wrong.
Again, anyone can set themselves up as a kitchen or bathroom fitter, so you need to be very careful.
Do not use a company like Wren (just do a search for all the bad reviews) as you have not control on how good the fitter will be.
Also, I wouldn't use sites like Chekatrade because the reviews can be fake, but if someone has hundreds of good reviews, then they are likely to be ok.
Some local authorites have recommended traders. Kent does:
https://tschecked.kent.gov.uk
There is also Which Trusted Trader, although I have never used them, so you need to check how they vet the traders:
https://trustedtraders.which.co.uk
If you do find a good kitchen fitter, then you can buy your kitchen form DIY Kitchens and get them to fit it. There are lots of threads about then on here if you do a search:
www.mumsnet.com/talk/property/5112933-diy-kitchens
https://www.diy-kitchens.com