The big advantages with Howdens are:
1) They have plenty of retail/trade units in their network, typically at least one in every main town/city. Locally to me there are 3 across 12 miles.
2) They carry a lot of stock for immediate or next day collection and delivery.
3) They have no problem with quick exchanges, returns or mid fit running modifications to design.
4) You can go in, get the design done/checked, agree the price and take the kitchen away all in the space of a couple of hours.
5) Units are pre assembled bases and you just add the doors and any specialist internals.
6) Quality is great overall.
Howdens will deal with normal retail customers but it is really structured to operate through the trade and via their fitter / builder network. You won't achieve great prices with them unless on trade terms.
Their outlets don't feel like a retail store and are little more than a trade counter for visitors.
If you have a trade acount then prices are very competitive and often well incentivised. If you find a good local fitter on their network who doesn't mark up too much from trade to retail, you can achieve a very competitive price.
Wren give the national retail showroom experience and offer heavily subsidised credit terms. Their kitchens are good quality and overall the fitting teams do a great job with any problems rectified once identified.
It's a numbers game for Wren, they supply a lot of kitchens and so will attract negative reviews. Like many large retail stores the majority of customers will be happy. Even John Lewis attracts a much higher degree of poor reviews these days, despite offering one of the best shopping experiences.
Using local fitters or independent kitchen showrooms (supplying from a national joinery like howdens) or a smaller local independent joinery is often the best way to secure the best prices as a retail customer. It's easy enough to get a bank loan to cover costs rather than relying on interest free credit but on inflated prices.
I have found Wickes/TP (benchmarx) vs Howdens on a trade basis offer similar prices & quality, with the advantage to Howdens due to their local depot infrastructure to me.