Europe 2017

I would start by checking transportation connections between some of the more exotic places. We've done a couple of trips with connections like that; and it can be surprisingly cumbersome and/or expensive. London to Paris? No problem, of course; but, e.g., Split to Coventry? Who knows. Skyscanner.com or kayak or one of the other similar sites are good places to start. You might find that it would be more economical to group your UK visits together; then buy an open jaw ticket to fly home from Croatia. Or, maybe not.
I'd love to know what you find out about Malta. We're considering it for our next trip. I already discovered that getting there from California is not what I want to attempt. We'd plan to make it part of a longer trip, but at the moment don't know where else.

I'm also concerned that you may lose more time than you expect making some of those moves, but they may be fine, Just do some preliminary are checking on rome2rio.Com to get an idea of,what you'll be facing. Given the hassle you may experience getting to and from Malta, 3 days seems a pretty short visit.

My second thought is that with Mykonos and Santorini back-to-back, you have a solid week of intensely touristy destinations. But you may be fine with that atmosphere.

The good thing about this is that except for the ending stay in London, you are staying at least three nights in each place. The transportation connections will probably have you spending close to a full day getting from place to place though, and it's likely to be very expensive. A lot of these places may not have cost effective or convenient connections, definitely research that. I would not try to combine such far flung and hard to get to locations on one trip, but it's your itinerary so if you really want to do it then go for it.

some good points.
Maybe I need to really decide what my priority is and either stick with the Greek islands or Croatia.
I always want to cram stuff in but as I am getting older I like to be able to relax and spend more time in one place.

Iwoof,

I like your overall philosophy of staying places for a bit, not always rushing around. Personally, I think I'd rather be in a coastal area on the mainland rather than an island, just for flexibility, especially if you plan on staying there for a few days. I like what I've heard about Split, in that its a city with a life of its own instead of just tourist based like Dubrovnik. Good luck fleshing in the details.

It currently possible to fly from Dubrovnik to Birmingham Airport but not Split. Not sure how you could easily get to either from Valletta though.

I have been to all the places you listed except Malta and Coventry. Unless you just love the beach, I would cut some days off Mykonos, Santorini, Malta and Split. All are great place, but you can do them in one day. They are great for visiting on a cruise. Santorini is my favorite, but one or two days is all you need. Dubrovnik is great, but again one or two days at the most. Split one day even if you take a trip to Trogir.

www.skyscanner.com is good for researching one-way flights in Europe. It shows no direct flights from Malta to Dubrovnik and the connecting flights often have a substantial layover.
Malta is a very interesting island, with lots of medieval and WW2 history. It doesn't have much in common with islands in the eastern Mediterranean.
If you are flying from North America a multi-destination search function might help you. At a guess, you might save both time and money by flying into Paris and then home from London, combining all your friends/family time in the UK into a single block (or do it in reverse.) These are not one-way tickets; you have to use the multi-city search on many travel websites.

I would suggest logistics like this to avoid time wasted with poor flight connections.
You will have more direct flight options to/from Paris, London and Rome.

London 4 nights
Mykonos 4 nights + Santorini 3 nights
Paris 5 nights
Dubrovnik 3 nights + Split 3 nights
London for Coventry England 3 nights
Malta 3 nights
Rome 2 nights

Ok so it seems Malta is a bit difficult.
If I was to cut Malta and add it on to the greek islands or Croatia would that make more sense?
I was thinking I could start the trip in Coventry with family and then head south, to paris and then London before I fly home

Coventry 3 nights (could fly from Birmingham or else do a few nights in Manchester)
Mykonos 5 nights
Santorini 3 nights
Dubrovnik 4 nights
Split 3 nights
Paris 5 nights
London 5 nights

For Greece and Croatia, you will have more direct flight options if you fly to/from London and Paris.
From Santorini/Mykonos to Dubrovnik/Split may involve lengthy connections in Athens or Zagreb.
That is why I suggest ... Coventry >> Greece >> Paris >> Croatia >> London