Re Spain only visit one city not the two listed . for just 4 days.... which is less if you include arrival and departure times .... you will not have much chance to see one let alone both.
Both cities are very different. Having given us some clues as to what you want to do in Wales and the Alps, but nothing about Spain...what are they?

Wow - 5 countries in 17 nights. Honestly I think you need to cut down the number of locations. I know you're tempted to do as many places as possible but sometimes less is really more in terms of pleasure and cultural understanding. You'll be spending alot of time in transit so you won't have full days in each city (1.5 days in Madrid & Barcelona). You definately won't have time for day trips as you've indicated. You'll also be spending alot of $ on transportation. You could easily spend this entire trip in Italy visiting Rome, Florence, Venice and the Italian countryside since you've indicated this is an interest.

Spain, I like both cities. I'd check temperatures, Madrid can get pretty hot. Barcelona is probably the one I'd send younger people to (I just sent my hairdresser there, who loved it). But the two big museums in Madrid are both world standard.

Some of your plan borders on impossible - just borders on it but pretty close.

For instance:

>>Day 2 - Holyhead (coasteering) -travel to London via train<<

Assuming you will be coasteering very close to Holyhead/Anglesey it will still take most of the day. Then it will take about 6 hours to get to London by train.

When you factor in travel times you will have 2 days in London, 2.5 days in Paris, 1 day in Lucern1, 3 days in Rome and 1.5 days in both Madrid and Barcelona.

I wouldn't . . . But that's just me.

Thank you all for the replies.

Ribeirascara, I think I will take your advice and pick either Madrid or Barcelona. We just haven't really done enough research on the country yet to figure out exactly what types of things we want to see while we're here.

CathyM, I know it's a lot of distance to cover but we both are dreaming of going all the way from the UK to Rome. As I just mentioned in my other reply, we are going to cut down either Madrid or Barcelona and spend 3-4 days in one of them (with a possible short day trip).

janisj, I've done coasteering before, and it only takes a half day (either in the morning or in the afternoon. The first few days of the trip may need to be altered depending on if the adventure company that offers coasteering will be planning a morning session or an afternoon session. Also, I believe the train rides from Holyhead to London are about 4 hours long, so we could do a coasteering session in the morning and be in London after dinnertime.

I agree with many of you that the best way to enjoy your destination is to slow down and enjoy it, knowing from experience. But so far, this itinerary has things that we both want to do and agree on considering our time and money constraint.

"Also, I believe the train rides from Holyhead to London are about 4 hours long, . . . "

Nope. The shortest option is 4hr 20-ish min -- but that is only offered twice a day either too early for you or at 6:19PM. The rest are longer from nearly 5 hrs to almost 7 hrs. (of course the schedules could change)