Depending on where you go you may or may not need this. But if like most people you are escaping to the sun then this is essential.
It can be very expensive to buy but you will find it even more expensive to buy at your destination so best get it before hand.
Get a good one with adequate protection for your skin type and climate. It is essential that you check both the UVA and UVB protection rating, most people go on the SPF or Sun Protection Factor alone which only tells you how much protection is provided against UVB rays. The protection against UVA should be denoted clearly on the product.
Nothing worse than being sun burnt on holiday and having to put up with the discomfort for the rest of your trip.
After sun is useful even if you don't get burnt to help your skin recover after a day of sun worship-ping.
Mosquitoes are annoying at best, sometimes the local brands will be better but best to take some with you to keep from being eaten alive. You may also need a plug in mosquito killer for the hotel room at night if you don't fancy waking up covered in bites. It is important to do your research, depending on your destination you may need repellent specific to the local insects and not all repellents will be effective.
Apart from looking cool you don't want to get sun stroke so if you are walking about you will need a hat. And to avoid constant squinting get some sun glasses this will also help protect your eyes.
May seem like overkill, but by this I mean things like re-hydration treatment. It's easy to underestimate how much extra fluid you will need to drink when out in the sun, if you get sun stroke you will feel terrible the following day.
Pain killers are another essential item for those mornings after.
Also on the list of items to have but hopefully never need is some kind of diarrhea relief.
Camera charger is essential if you are going away for more than a few days, nothing worse than running out of juice half way through your holiday and missing those photo opportunities.
Keep a note of your hotel address, insurance details, tour operator help line and your banks lost or stolen card line. You may also want to include a note of your passport numbers in case you should lose them. Hopefully you won't need them but your hotel address is useful to keep on you if you need to get a Taxi. It is also a good idea to give someone at home this information too. I have put together a quick template you can download and print off in either Word or PDF format.
Not essential but it's handy to have something to separate your British from your foreign money.
Also optional and entirely dependent on where you are going. But to help dispel the reputation of the ignorant Brit on tour it is good to at least learn a few basic phrases. If anything you will get a better reception than if you go in shouting English at people.
You will be waiting around a lot so bring some entertainment. MP3 player or a book also useful for sunbathing and blocking out annoying people on the plane.
Good to keep handy in your hand luggage you don't want to miss any photo opportunities. Also make sure you have plenty of empty and spare memory cards (assuming you are not still rocking the old 35mm) and the camera is fully charged.
Handy for emergencies make sure it is fully charged but also check with your network what charges will apply while you are abroad. If your phone has internet capability make sure you either switch it off or don't use it unless you feel like paying a massive phone bill on your return.