In terms of exchange rate, or what you can buy with it?
In terms of exchange rate, the dollar is smaller than the pound, so when you exchange money, you will end up with a smaller number of pounds. But each one is worth more. How much more varies from time to time and you see the exchange rate varying from day to day. If you're spending using a credit or debit card, you'll notice when you see your statement that whatever you spend in pounds will be converted to a larger number of dollars. This does NOT mean your money is worth less, just that we use a bigger unit so the actual numbers are smaller.
As for what it's worth in terms of what you can buy for the equivalent of a dollar, that varies between things depending on what it is. For a couple of examples, food tends to be cheaper in the UK because there is a large agricultural subsidy to keep prices down and there is no sales tax on most food. But hire a car in the UK and you may be alarmed at the cost of fuel. I just went past a petrol station today and it was £1.31 per litre. That may not seem too bad, but remember a litre is only just over 2 US pints (around 1 3/4 British pints), so multiply by 4 to get a rough conversion to £ per US gallon.
Which is another thing about the UK - many things are sold in metric measure, so it may help to know some metric conversions. You might not be buying food by weight, but if you look around a supermarket, it might help to know what g, kg and litres ARE!