I used a Radical Storage location in Portsmouth, UK a couple of months ago. The cost was £5 per day per bag plus an additional amount for mandatory "insurance". The extra fee was between £1 and £2 per bag per day.
The way Radical Storage operates is that you book and pay in advance without knowing the exact character of the storage location. My storage location turned out to be a vape shop that had a strong odor of scented vapes. I was worried about what the clothes in my suitcase would smell like after being stored for quite a few hours in that environment, but there was no problem.
I've never really worried about the security of stored luggage. I don't travel with valuable items except my phone, which I keep with me. I try to remember to transfer my bottle of prescription medicine to my purse if I'm going to be storing my luggage somewhere, but I often forget to do that. Neither my travel clothes nor my luggage suggest that it would be worth the risk of stealing or rifling through my suitcase or tote bag. I do keep a cheap lock on my suitcase.
I'd prefer to use a locker, since I'd probably be able to fit my suitcase and tote bag in a single locker, which would likely be cheaper than the storage fee for two bags.