I think that the 4 mbps connection is quite fast. The 2 mbps one is a bit slow but still usable. I use my mobile data very rarely, but when I do, the 3G is pretty good for my needs.
I'm also in the UK, but live in an area where some houses don't even have twisted pair copper.
However, I'm also 1km direct line of sight from a mobile transmitter. So I use Three mobile's One Plan - £25 GBP for all you can eat data, incl. tethering. I regularly push 250Gb through my phone in a month, and get ~15Mbps down, ~8 up, with avg ping response of 65ms. That's good enough for me.
Oh, and its also completely portable.
That's pretty much the speeds I get on 3G here, and the ping is reasonable too (down to 30-50 ms sometimes). The only problem is the 3G plans - the highest you can go is unlimited with 10GB FUP, at about £15. After 6GB, the speeds drop to practically less than EDGE. That's the most expensive plan you can buy.
I considered my 11 up/1.5 down connection slow. Talked to my ISP, they couldn't help as apparently the line on my street was out of date. So I switched to a new ISP, to 45 up/10 down. I consider that to be fast. This is outside of Chicago, if that matters.