You are trading through a spread betting account against your broker as you might bet on a horse against your local bookmaker, not with the foreign exchange market so your losses are not tax deductible and nor are your profits taxable.
If you traded directly with the foreign exchange market through an ECN account, that could be classed as trading and your profits might be subjected to income tax or capital gains tax. HMRC don't appear to have developed a policy on this yet. I know professional financial traders who disclose profits as capital gains so their tax charge is limited to 28% maximum and as low as 18% and other traders who disclose their profits as income so they can pay get a mortgage or pay pension contributions to keep the income tax charge down.
If the tax office saw your trading as a hobby as opposed to a properly run business, they would probably disallow your losses even if you were trading directly with the foreign exchange market. Take a look here
for information on hobby trading.
The link from the HMRC site you gave me is about foreign exchange gains and losses. The profits from exchange transactions, particularly one off transactions, are normally subjected to Capital Gains Tax. You could not offset an exchange loss like that against your income from self-employment.
For example, an individual may buy a house abroad and sell it ten years later for a cash profit in the local currency. If the exchange rate had moved in his favour over the ten years, he will also make an exchange gain which is subject to capital gains tax along with the cash profit on the property. If it had moved against him over the ten years, the exchange loss would dilute the cash profit and the gain on the property disposal itself.