Hi there.
To be honest, any competent independent can do this, but if you are struggling for guidance, try this link;
http://www.honestjohn.co.uk/directory/ and I'm sure you will find someone local to you.
Try if possible to hear the engine running before purchasing it (or make sure it has a guarantee that it runs, if already extracted).
I'd also get various service items changed prior to installing, such as the belts, water pump & an oil & filter change (once its in), as this is much easier done with the engine out (so cheaper in labour terms), plus will give you peace of mind for the future.
HTH
Paul