I would try Perugini and Visini in Italy.They supply parts to the Italian trade.I would also get your tubes from Peter Hambrusch.You will have them in a few months of your order.
The English gun trade is very difficult to deal with and verging on a cottage industry.Ian Clark is a very Busy man dealing with the Rigby Rising bite parts.I know he would take your order but would hate to think that in a years time you still don't have all the parts to build your rifle and I never warned you.The same goes for Arthur Smith with the tube's.Unless Arthur has the tubes in stock,which is highly unlikely you could be waiting a very long time for your parts.Westley Richards use Hambrusch Tubes and they shoot very well.
It's all very well having stuff made in the UK.The finished Rifles are lovely but getting there is a nightmare.