It really depends what your trying to improve, also your best suited to buying a router with the same chipset manufacturer as your cabinet/exchange.....being sky its more than likely Broadcom based, so if you look for a Broadcom based ADSL router with the functions your looking for then get that one, there's that many to choose from...for example the TP-Link WD9970 is a nice cheap router at appx £35 and works well on Sky as broadcom based, or up to any of the top of the range Netgear, Dlink, Cisco, etc....over £300 lol
Some buy routers just to put DD-WRT or Tomato for example on them, really depends what your after, but almost anything is an improvement over the sky hub, I have the SkyQ hub and its still rubbish.