I am only speaking from personal experience here, so bare that in mind and I am in Australia, so things may be somewhat different in the UK.
First suggestion – be very sure what you are looking for. Email contacts and don’t be afraid to ask the tough questions. To me ensuring that a sperm donor was in good medical health and was willing to sign a legal sperm donor agreement were most important.
Meet in person next and again, discuss EVERYTHING!
In Australia, unless there is a legal agreement, the donor father can have an ongoing contact with the child and the mother may insist on child support payments.
I feel lucky that I have found a sperm donor who is on exactly the same page as I am. We discussed everything prior to moving forward, we swapped medical information and we have a legal agreement in place.
If you want to chat further – email via my profile Tori2
hope this helps