Unless the router you have has the option to disable the adsl modem, and has an ethernet uplink port (sometimes called WAN) then you won't be able to use it with cable. Can you operate the router in access point mode rather than router mode? If so you can buy a normal cable router, and keep the current router as a wireless access point.
Unless you have ntl or telewest, you have to have a BT line [although it could be a Bulldog line] so stuck with the £11.50 a month to BT.
If you sign up with ntl now, you can have 1meg broadband, phone line and base pack digital TV for £30- so saving some money and getting an extra service
If you want to stick with 2meg BB you can have this from ntl for £24.99 (not sure if they are doing special offers at the moment)- no need to have a phone line from ntl if you have this (or have a phone line at all infact!)
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oh and with regard to your other question-
The difference between crossover and straight cables is given away in the name- with a crossover cable, the little cables inside actually cross over (you can see this in the connectors at the end). Crossover cables are used for connecting two pcs together, and straight cables for connecting to routers/ switches/ modems. It is worth noting that some modems and routers (and all apple macs) will auto-negociate, so if you connect the wrong type of cable they will deal with it