Is this what you are looking for:
uk.iherb.com/Source-Naturals-L-Tryptophan-500-mg-60-Capsules/7276If you order that and select 'global mail' for shipping (£2.88) - NOT DHL - the total is under £15, so there wouldn't be any customs duty to pay.
I buy most of my supplements from healthmonthly.co.uk. They have the things you mention in Swanson brand. I don't use Buhner, but I use lots of Swanson supplements which I have always found to be effective.