Like silver mentioned, it all depends on what you want to spend.
The cheapest ASIC option is a Bitmain Antminer. There are several types. The most common model is the 1.6 Gh/s U2. It can be overclocked up to 2 Gh/s but be prepared to keep a fan on it. You can run one from a USB port on your PC, but if you want to run more than one, I'd get a powered USB hub. They appear to be selling for £15-£20 on Ebay.
Before you sink funds into multiple Antminers, you can take a big step to a 32 Gh/s Rockminer. They seem to be scarce on Ebay right now. Here in the US, they've been selling for as low at $25 US. They do require a separate power supply like this...
http://www.amazon.com/12-Volt-Power-Sup ... s=12+v+psu.
The most efficient way is to power them from a 6 pin PCIe power cable off a PSU with a cable like this...
http://www.amazon.com/PCI-E-6-Pin-2-5mm ... wer+supply
If you use one of these, be sure it's made well. I soldered all my wires to the pins. I have several I'm ready to let go of, but I'm in the US. See my offer here...
http://www.dunadd.co.uk/seti/forum/view ... =25&t=2683
Next step up is the 100-110 Gh/s Rockminer. There's a few on Ebay. They require a 6 pin PCIe power plug, they same type you plug into a GPU.
From there, are the multi-blade Rockminers like the R3. They are essentially 4 separate 100-110 Gh/s miners. Each one has a separate USB port and requires it's own 6 pin PCIe power.
Be aware, they do use power and the cooling fans do make noise.
The 32 Gh/s Rockminers use 50-60 watts and are pretty quiet really.
The 100-110 Gh/s blades use about 120 watts each and the cooling fans do make noise.
I have a Rockminer R3 with all 4 blades connected to a Raspberry Pi running BU, all powered by a Corsair HX 850 PSU. It pulls over 500 watts when it's running peak.
Let us know if you have any questions.