Aaaaactually a big reason for them not being released would I imagine be that there isn't much demand there. About 0.5% if I remember right of Telewest's customer base take 2Mbit at £50 per month.
If you live in the right areas you can get ADSL up to 8Mbit right now, but at a fairly hefty price, and up to 6Mbit in Central London for about £100 per month.
Bonding 2 DSL lines together will cost at least the price of the 2 2Mbit lines - cheapest business product price will be over £120/mth, not including any further charges for the Multilink PPP. One ISP at the moment is going to be doing this, Nildram.
You can 'load balance' between any connections if you have the appropriate hardware.
Considering that BT have only just released 1Mbit for home users I can't see there being a big rush for speed just yet. It's something I would
very much love to see but at the moment while the biggest selling products are 150k and 600k, or 512k in ADSL's case there's not really any demand to speak of there for 2Mbit and up. Simply, most people are happy with what they have, and don't see the point in paying £50 a month for an internet connection. (I can remember paying £300 in a month for a 1p per minute service...ooops

)
A couple of ADSL ISPs offer 2Mbit connections at £37.50 - £44 a month, these are however contended much more than ratios found on the £65 a month+ business packages and cable.
If you really want 2MBit that badly encourage the demand. The other issue is as was quite rightly said, what's the point in giving someone who already downloads 10GB a day the ability to download another 10GB a day - if you are losing money already on current traffic why lose more?
I've posted at length about this in the past. At some point for speeds to go much higher and sustained high speeds to be achieved there need to be controls. You can't expect to have 2, 3, 4Mbit and achieve it constantly without transfer limits/traffic shaping being in place. With current pricings in the UK, and how used most are to apparently contention free networks something will have to give before the speeds can go through the ceiling.
I download maybe at most 25GB a month, and I'm prepared to accept a reasonable traffic limit to keep a service good, my usage is 'bursty' in that I want high speed to do things quicker, more than to download more. I may well be in the minority in that regard so it comes down really to 'are you prepared to compromise'. Following a poll I brought up on this subject on another site the answer seems to be no.
Wanting/demanding 100% of throughput all the time, unlimited transfer, and a low price isn't going to work. Something somewhere has to give, a sad fact really but the way it is