When it comes to choosing your outfit for a wedding you need to consider what type of wedding it will be as this can impact what you may want to wear. If it is a fairly low key event then you will not want to go too over dressed as the bride and then bridesmaids should be the ones that stand out of the big day.
If the wedding is going to be mostly outside then you may have to think about the weather. unfortunately the weather in the UK is fairly unpredictable and therefore you will need to plan for every occasion. Make sure you have a coat or a shawl that you can put on should it go cool or start to rain.
Also with outdoor weddings you need to think about foot ware. It may be that most of the area is grassed, if this is the case you may want to ditch the heels that will most probably just sink in to the ground and make it very hard to walk.
For weddings abroad, you may be looking at rather warm temperatures. It might be a good idea to have more than one outfit so you can get changed for the evening when the temperature may get a bit cooler.