Definitely a set of white bluemel fenders. I'm not sure how available these are, but a shop local to me has them. Shipping across the pond would probably make that an unattractive alternative.

On the parts...do you just want a derailleur that fits the other parts on the bike, or are you looking to restore this to what it "should" be?

If you want a reasonably appropriate derailleur that fits the other parts, you're looking for a Shimano 7sp gruppo 105, or tricolor. Good news, they can be had on ebay for a song. I'll dare say a member here can help you out with one. I'd send one myself if I had one in reserve.

There is absolutely nothing wrong with the Shimano parts on there - they work quite well. In fact they likely work better than the "correct" parts I'm going to recommend. To me, that bike is just AWESOME. There aren't many bikes I've come across as nice as Jack taylors are, and I'd probably build it up with period appropriate higher end parts. I'm not sure what the hills are like near you, which would determine some of what I did, but I'd probably go with Campagnolo nuovo record and a triple crank on front. That would likely be the time appropriate direction, though 84' also saw the release of shimano's dura ace 7400 group. A 7400 group would not be out of place here at all.

There are a lot of different directions you could go here. The star of this show is the frame, and regardless of parts, the frame is a beaut. I'd also consider Suntour superbe. I might even Frankenstein it (using a mix of parts)...a TA triple crank, campagnolo NR levers, tektro dual pivots, Suntour cyclone derailleur, or maybe Shimano deerhead. You could even modernize it with some nice silver campagnolo centaur 10 speed.

It really depends on your needs, wants and budget!