Here's some BC specific places.. there are also always collies at Dogs Trust, RSPCA, Blue Cross etc.
http://www.bordercollietrustgb.org.uk/
http://www.valgraysbcrescue.org.uk/
http://www.wiccaweys.co.uk/
http://www.thebordercolliespot.com/
http://www.fostbc.org.uk/
http://bordercolliesneedinghomes.weebly.com/