For small quantity you will probably need to go to somewhere like Ikea or choose some of the off-the-shelf stuff from one of the large DIY places.
I have just gone through something similar after having a brick-built cupboard removed from our kitchen, which left a distinct lack of storage space on that side of the kitchen. I wanted 3 units – larder unit, bin cupboard, 800mm wall unit & some worktop.
For us, the size of units was quite important to get what we wanted to fit in the space.
I ended up going with DIY-Kitchens, but they have a min order quantity of £750 ex. VAT so I had to make the difference up with end-panels etc. which I wasn’t actually that bothered about having.
You will probably be too far under the min spend value for it to be worth topping it up.
Wren is the same with a min order quantity & a distinct lack of interest in providing anything that wasn’t stupid expensive.
I did buy an off-the-shelf plinth from B&Q to space the larder unit off the wall – I was quite impressed with the amount of stuff in stock & while not the best, it seems OK quality.