Personally I wouldn't feel comfortable going into work in a beach dress so here are some more sensible ways to stay cool:
Cool drinks are a great way to refresh and revive in the heat - avoid sugar-laden iced tea and frappucinos in favour of chilled fruit smoothies, fresh juices or Leon lemonade (which you can make yourself with water, fructose and freshly squeezed lemons).
Instead of ice cream freeze mini-sized fruit yoghurts with a lolly stick stuck through the lid of each one to make frozen yoghurt lollies. You can also make lollies by freezing fruit juice and make your own fruit frappe as an alternative to high sugar sorbets (see recipe below).
It may be obvious but avoid eating hot meals especially meat - these will heat you up whilst you eat them and in the case of meat/fish the protein will stay warm, continuing to generate heat, for quite a while after you've eaten. Go for water rich salads instead.
For instant relief put your wrists under a cold tap, or if you're at home your feet in a basin cold water. At both these areas your veins run closer to the skin so it's easier to cool your blood at these points.
Make sure you up your water intake to counteract increased sweating, especially if you're exercising in the heat. Counteract water lost at night (more than most people realise) with a glass of water on waking.
If you've got a long commute or travel on the tube make sure you have water with you and layer your clothes so you can stay cool on your way to work and then cover-up when you get to the office.
And if it all gets a bit much, just remember the summer doesn't last long here!
Jamie Olivers pineapple and grapefruit frappe:
2 ripe pineapples peeled and chopped
3 grapefruits, halved and juiced
Sugar to taste (or sweeten with agave syrup for a healthier option)
Whizz up the pineapple in a liquidizer until smooth and pass through a coarse sieve. Add the grapefruit juice and stir in sugar to taste, remembering the sweetness will lessen slightly when frozen. Freeze for 2.5 hours stirring every 45 Minutes til set, then serve.
This recipe will work with pretty much any fruit combination and may not need agave syrup/sugar depending on the selection. A lazy alternative is to keep chopped fruit and berries in the freezer. Then you can take them out whenever you fancy, defrost very slightly in the microwave and use a blender to blend into a slightly runny sorbet. I like to do this with mango and raspberries.