Those are the correct parts, but one is N/A. It appears Moss has depleted their stock of individual exhaust pieces as they try to sell complete kits. If the rest of the exhaust is SS you should probably stay with that, although mild steel would work for many years. I'd call BCS and see what David says:
https://www.britishcarspecialists.com/html/about_us.html
Downpipes come with the flexpipe welded to it. You'll need to get your car on (very solid) jackstands or, preferably, a lift. Don't take any chances--quite a few people get squished under cars every year but, yes, you can do this. Since your downpipes are already separated, you just have to remove the 6 nuts on the exhaust manifold flange studs (3 on each). They take a 1/2-inch deep socket. It appears your exhaust is fairly new, so you shouldn't have much trouble getting the nuts off. Just try not to bunge the threads too much when you pull the downpipes off (wouldn't hurt to chase the threads before you put the new pipes on).
When you fit the new pipes, put some antiseize on the rear (by the flexpipe), then insert into the muffler. Easiest to do the inside pipe first since it mates to the rear manifold flange, then the outside. Wrangle the pipes so the mate up to the manifold and into the muffler opening just so, then install the nuts.
Note: the 'correct' nuts are brass; either buy new when you get the downpipes or I use steel with lots of antiseize. Recommend you do not reuse the brass nuts, they distort and the threads wear so they don't hold as well. You'll need flatwashers under the nuts; I recommend grade 8/hardened so they don't distort and lose clamping force.
Don't tighten the clamps at the muffler opening until you've replaced and adjusted the hangers, else you could bind the pipes. Then, when everything is the way you want it tighten the hanger hardware and finally the clamps.
Start the engine--be VERY sure you are on solid jackstands/lift--and check for leaks. If you can't get a good seal you can try some high-temp silicone around the pipes.
So, you'll need:
- 2 new downpipes (one inner, one outer or back/front)
- 2 manifold-to-downpipe gaskets
- 6 new brass nuts
- 6 flatwashers
- new hangers
- assorted nuts, etc. for hangers, clamps, etc. (can re-use old)