I did make enquiries from one of these companies, when I was trying to sell my house two years ago. It was all done over the phone and quite interesting. I had put the house on the market in July 2018, just at the start of an intensely hot spell. Had very few viewings, reduced the price of the house I had thought the EA had valued it much too high), still nothing happened, so I reduced it again. Then I phoned this company.
What they did was to look it up on Rightmove and then offered me about 80% of what it was being offered for on there. I did ask what would have happened had I contacted them when it was on at the very much higher price - would they still have offered 80% o fthat price, which would have been nearly what it was on for at the lower price. Never got any reply to that.
So, perhaps you need to get as high a valuation as you can on your house, by a local EA, get it on Rightmove, and then contact these companies. They say it can all be done with a week or two, but from I actually hear, even with them, it can take several weeks to complete.
If you do go down this route, it would be interesting if you could let us know how you get on with them.