On the speed issue, TalkTalk can offer ADSL2+ since they have their own hardware, TalkTalk offer ADSL2+ at more exchanges than BT Wholesale do.
Getting connection stats from existing router and feeding them into www.farina1.com/adsl
will help you see if their estimates of speed improvements are better.
Important our speed comparisons are based on peoples votes, NOT actual tests of lines.
On Simplify Digital better just another sales site, now if you have an attenuation of 47dB now then you wont go much faster, if you have 40dB attenuation and are only on IPStream (ADSL) then you could well see 10Mbps from TalkTalk or Sky