Denbies may be rather atypical, as I believe their aim is to cater for the coachload, and the age profile of those tends to be higher than average.
Not all wineries are set up to receive casual visitors, and that includes probably the majority of crémant producers in Burgundy. I have visited one but had to pay. I don’t generally have a big interest in Crémant de Bourgogne, as many are drearily pedestrian, though good ones certainly do exist. Conversely, if you visit Alsace (another major crémant producing area) you will likely get a free tasting if the place is open, which it may not be if you didn’t book ahead.
I guess what I am saying is that these things vary from place to place, even within a country. The business has to make a profit, and people arriving by coach frequently leave without buying anything at all.
Round this way (Hampshire), tha majority of producers do co-operative tastings. They will take it in turns each year to host an event, where all the other producers will attend. You can then taste every wine on offer, which is almost always every wine they make and have for sale. Since they also arrange various local artisan food producers too, as well as the usual hog-roast etc., along with guided tours of the host winery, then I would regard the £10 it costs as money well spent.
On the pricing model, most of the points have already been well made. Mostly it is the market at work, and English producers would be foolish to get in a race to the bottom such as happened with Prosecco. Though one point not mentioned so far is that fruit yields in the UK are substantially lower than from the other areas mentioned. For example in 2016 and 2017, average yields were around 4.5 tonnes/ha, which is generally reckoned to be below what you need to be economically viable.