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I am based in the UK and wish to open up a Forex Trading account.

What currency will my account held in?

How does this affect (if anything) the pairs I trade?

If my account is in USD, and I only ever trade EUR's and GBP's - what further considerations are needed when calculating risk/reward, money management and so on?

You can held your account on multiple currencies. I'd recommend USD, EUR, CHF.

You can open an account based on any currency you wish (depending on the broker). I'd suggest USD.

Also, risk and reward as well as money management are calculated as percentages and ratios...the actual currency used does not matter. For all the numbers care, you could be using sheep!