Hi Elan G0UUT
The SmartSDR page at http://www.flexradio.com/amateur-products/flex-6000-signature-series/smartsdr/ has a "Requirements" tab that lists what you need.
As Amazon.co.uk offers different stuff than Amazon.com (USA) a recommendation isn't going to be very useful.
Perhaps you can check with some of the other UK based SmartSDR guys for locally available machine recommendations?
If I recollect well my first UK built computer was a Ferranti Argos-Pro dual bootable as either a CP/M or MSDos machine. Perhaps there is one still running at the Bletchley Park museum, but that puppy won't do SmartSDR!!