How are you planning on getting around / when are you planning on going? Is there anything in particular you're interested in?
If you're flying, I'd suggest going with the order that makes the most economical sense flight wise with London either first or last as it is outside the Schengen zone and as a non-European you will have to go through a longer customs process, but with trains /busses / cars I would either go north - south or south - north depending on time of year (it can get quite hot in the summer in the south or be quite cool and rainy in London in the winter)
I see that you're from the Philippines so you may want to allow yourself a couple of days to recover from jetlag, in which case 4 places in 3 weeks is actually quite tight, especially if your flight time falls within those three weeks (+3 days of travel lost), and taking into consideration travel time between locations.
If you can give us more information on what you're thinking we may be able to help you plan your trip a bit better. Best of luck in any case.
Plane from UK to the next European country. We'll only visit one city in Spain so just a few days there perhaps. Just not sure which country I should go to after UK. Ideally, we will fly in to UK then fly out from UK. Was thinking UK to Italy then France and Spain?
Ah, if you're flying to the UK, i might consider bus/train to France - Paris - then continue from there. The eurostar is only a couple of hours and you will avoid flights that way. As it goes from central London, it's a pretty easy trip.
From there, it would depend on time and economic sense. You can take overnight trains or fly depending on personal preference, but it is quite easy to get around europe. I still think it's quite tight and you might be better focusing on one country / area in comparison to bouncing city to city, but ultimately, it's your trip and it will depend on what you want to see / do.
In April my gf and I are going for 3 weeks;
They all have tons of stuff to do! you would be hard pushed to get england involved esp as some times they have troubles at the docks then driving a car in london is a nightmare with parking and congestion chargers....
Begin from London, then Barcelona, Paris, and as a final point travel to Rome and Venice. I can endorse strategy for your journey you can discover the most exciting Sights that is suitable to holiday in every city.
Start with London, then Paris, then Barcelona, and finally travel to Rome and Venice. I can recommend plan your trip with the help of -snip-. There you can find the most interesting places that is advisable to visit in each city.
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In France, you can also visit Bordeaux; there is an awesome dune - Dune du Pilat near it. City itself is also nice.
I didn't like Venice much, but hope you will have fun there.
London should be your last or your first stop. Porbably better the first option. Then, Paris, Barcelona, and Italy. That is what I would do