alright- i'm not an accountant and guya & lizzie will probably correct bits. but in laymans terms:
the money i was transferring were US income that had been invested in mutual funds in the US. when i sold these, i had to pay long term capital gains taxes in the US.
when i moved them to the UK, i had to pay UK capital gains taxes as well. now, the capital gains taxes are different here. first off, there's no long & short term capital gains for stocks/shares/mutual funds. secondly, everyone gets an allowance for capital gains. ie- i didn't have to pay tax on the first £85000ish of capital gains income(that was the allowance in 2005). it also gets more complicated by the exchange rate on the day you bought & the day you sold the funds. by manipulating this (with the help of my accountant), i was able to minimize my UK capital gains liability. you also get to deduct capital gains taxes already paid in the US (hence avoiding double taxation that you point out)
so, even though I transferred a large sum of USdollars to the UK, the actual profit was low, and the actual tax liability. in my case:
Total Profit: £10,440.89
Amt above Capital Gains Allowance: £1,940.89
US Taxes Paid $2,271.89
US Taxed Paid (in £) £1,247.04
UK Taxes Liability £693.85
UK Taxes Paid £152.65
This £693 was taxed at my standard rate- which was 22%
a few months after i transferred the money from the US to the UK, i recieved a letter from the inland revenue informing me that I would need to fill out a tax return for that year. i forwarded my US tax return, P60, P11D to my accountant, and viola- paid my taxes.