service does suffer when prices are squeezed -
Working for the best of the big six, I'll leave you to your own thoughts on that one, and as a network specialist normally working with MPLS/private circuits etc, there are a few thoughts I can leave you with here.
Yes, there is indeed a squeeze on service from all operators - having a fiercely competetive market with an emphasis almost exclusively on price - a demand driven by the consumer, means that something somewhere has to be cut.
As it costs largely the same to provide a DSL service, if one operator is selling it for £10 a month and another for £30 a month, guess where the gap came from. When it comes time to renew contracts, not many people put service over monthly costs and then complain when they don't get thier 'moneys worth'! Serously though, service teams cost, and the people working them expect a decent salary, and consumers just aren't willing to part with the extra.
Performance - ADSL runs over telephone lines - they weren't designed to deliver high speed data, and sometimes they work, others they don't. Whether it works or not depends on the individual routing from the exchange kit. If you're line won't deliver 8Mb, it's probably down to physics rather than your ISP being malicious - above all remember that you're paying a very small sum of money for what would have been a very expensive service not too long ago.
If you really need a reliable data flow, have a leased line installed from an ISP - it will only cost about £650 per month for a 2Mb service - I remember installing these abut four years ago to companies who couldn't yet get broadband and it cost £1200 per month - kind of puts broadband pricing and service into perspective : )