Much has changed since the investigations of the 1990s: huge pubcos now dominate the market and use that power to demand huge discounts from brewers whilst charging their tenants full price. They employ other tricks such as upward-only rent reviews and five-figure fees off breweries for even being considered as a supplier. They rarely permit their tenants any free choice in beers ("guest beers") and their policies mean that small local breweries have little chance of selling to local tied pubs.
CAMRA is campaigning for all pubco tenants to be allowed free choice of at least one beer.