As recently as last year I was denied boarding a VT service from EUS because I didn't have a seat reservation with my SVR.
This was during the bout of heavy snow which led to many cancellations on the line.
I believe they had a quota in place for passengers without a seat reservation as every few seconds the RPIs were shouting out consecutive numbers, counting up. Unfortunately I was further up the ramp and reached the ticket check after they had counted to 30, after which I assume the quota had been reached and all other seats had been reserved. The service must have been at least half full before the passengers for my service arrived, as we were boarding the train which had been cancelled half an hour prior and had been 'merged' with the one I intended to get.
Personally I dislike the policy in several other European countries of mandatory seat reservations on many, usually fast suburban and intercity, services. Many times I have had a visit from a guard whilst travelling across the Hungarian Plain and the like and have been hit with another 20% of the ticket cost in seat reservation fees, despite the train being nigh on empty, my (perhaps cheeky) offer to stand -- and the fact that had I travelled on a service that departed 25 minutes earlier but took only 5 minutes longer to reach my destination, I wouldn't have been charged!
I have also been charged a seat reservation fee when travelling from Ferihegy into Budapest, Domodedovo into Moscow, and into Belgrade. In the first case the fare was the equivalent of £1.10, a few kilometres yet I was charged 650% of that in seat reservation fees, the same as a passenger travelling the entire intercity journey!
I think an introduction of mandatory seat reservations in the UK would be a major cause of more public grumbling and additional inconvenience. Not to mention chaos on currently unreservable peak time commuter routes. At least if one hasn't got a seat reservation, there is still a chance of sitting in a free seat in a reserved or unreserved carriage -- and passengers with seat reservations ate pretty much guaranteed that seat.