BTW, depending on how much cash you have in different currencies, it may be worth considering a "dollar cost averaging" strategy (despite the name it doesn't apply only to dollars).
Usually it's aimed at investments over reasonably lengthy timeframes, but in a volatile market it may make sense even for a timeframe of a few days to a few weeks.
Basically, what you do is "chunk" your money up and "invest" (in your case, exchange) a fixed amount every period. That way, you won't do as well as the best day within the period, but you also won't get hurt as badly as if you picked the worst day.
An example (completely fictional numbers)
Day 1: £1 = $1.20; $20,000 = £16,666.67
Day 2: £1 = $1.25; $20,000 = £16,000
Day 3: £1 = $1.23; $20,000 = £16,260.16
Day 4: £1 = £1.19; $20,000 = £16.806.72
Day 5: £1 = £1.17; $20,000 = £17,094.02
If you happened to change all your money on the best day, you can see at a glance that you stand to make quite a lot more (£1,094.02 more) than if you happened to change it on the worst day.
But because you can't predict in advance which will be the "best" or the "worst" day, you decide you'll exchange 1/5 of your cash over each of the next 5 days. That earns you £16,565.51 - not as good as the best day, but £565.51 better than the worst day.
(As I said, this technique is more commonly applied to investments such as stocks or unit trusts, with funds being added monthly or quarterly)