Wheel upgrade advice

Have a look at the Cosine range from Wiggle, i have 2 pairs and am very impressed with them

Yep. Campag Zondas and Fulcrum 3 are the exact same wheel with a slightly different spoke pattern. Buy whichever is best value.

My mate has the exact same bike and went for Zondas. He's delighted with them.

I have pre C17 Zondas and they are very good. I also have RS81s and they are also good, but in my opinion not worth the premium over the Zondas.
Also, I believe the Zondas are very similar to Fulcrum 3s; probably because they come from the same company.

I forgot to say the ultegra rims are prety narrow and mine rode horribly with a 25 mm clincher.

I have had the same set for 2 years and even swapped them with the fulcrum s on the new bike . No problems with them so far. I find them to have better hubs than most and the best skewers by far. Maybe they are a bit boring to look at for some and a bit too quiet when freewheeling for others. Oh and I have continental gp4000iis tyres on as well which fit fine if you take care pushing them in.

These are great wheels. You don't need anything to convert them to tubeless, they even come with the tubeless valves (and a freewheel spacer if you are running 10 speed). I have hammered mine for 18 months on all sorts of surfaces, especially the local Surrey lanes - a true testament of their build quality. They are as true as the day they were purchased. I have run a small number of different tyres on them including tubeless (my current setup). Never have I had any issues getting tyres on and off even when running tubes. If you know how to do it properly you should have no issues. For the price they are light at 1680g. They are stiff and responsive. Your money would be well spent. If you are a member of British Cycling you will get 10% off at Chain Reaction (even if already discounted). In a word, bargain.

I had those Ultegra wheels and was distinctly unimpressed. They're not light and they weren't very stiff. They had a dull heavy feel to them.

I'd recommend you go for Fulcrum Racing 3, they're much stiffer and lighter - both differences you'll really feel on the bike. You can pull off the stickers if you don't want them. They're basically Campag Zondas but with more regularly-spaced spokes. Both are absolutely excellent wheels for the price and hold their value pretty well.

I'm running these at the moment and I like them. I've only just started running tubeless on the rear one  and it works a treat. Initially, I struggled with getting tyres on and off these wheels, but I found a technique that makes it a lot easier. Move all of the tyre beads towards the centre of the rim which gives the tyre just enough room to make all the difference.

Ultegra look good wheels for the price. I've seen them a little cheaper, just search. If you go tubeless, I would recommend 'stans' rim tape and sealant, it's excellent at ensuring a good seal. Tubeless tyres are no more difficult to fit than most normal clinchers.

I had the rear ultegra for a while and had a real nightmare getting tyres on and off it. I now know that anything tubeless compatible is likely to be harder to get clinchers on and off. Campy Zondas C 17 might be worth considering, they are very popular.

I believe the WH-6800 Ultegra wheels you were eyeing are tubeless-ready, too. Just in case you were eyeing that particular tire option.

I swapped the R501s on my winter bike for some RS31s.

It's a fairly simple process, but you do need a chain-whip and a Shimano/SRAM cassette tool to fit it yourself.  Your cassette is newish so you may as well just swap it across, plenty of videos on youtube if you can't work it out.

You'll likely need to reindex the RD after the swap, and possibly adjust your brake calipers - but nothing major.