Land: Scotland, Wales and England make up one island. When combined, it is called Great Britain. Northern Ireland and the Republic of Ireland make up another. Northern Ireland, when combined with the countries in Great Britain, becomes the United Kingdom. The Republic of Ireland is not affiliated with the UK. See map
Currency: This is a brilliant question, as there are differences. The Republic of Ireland uses Euro. Northern Ireland uses Pound Sterling. Source It sounds as if it is easy to exchange the two should you be visiting both areas. Scotland also uses the Pound Sterling, but like Northern Ireland, uses their own country’s printed currency. There is a good possibility that you may have trouble using Irish currency in Scotland and vice versa.
As for currency conversions, just do a quick search for one on the internet. When it comes to budgeting, the more lax the trip plans are, the harder it becomes. Personally, when I travel around multiple countries with different currencies, I worry less about the exchange rate and more on keeping keeping enough money on hand to cover costs before moving on to the next one.
Weather: As anywhere, this is a matter of opinion. Yes, the cold and rain are common in this part of Europe. My advice is to invest in a good trench coat that has a removable warm lining, as well a hood, as it can get a bit breezy.
Packing Pack clothes that can be layered and washed in a machine or sink. Pack as lightly as possible. Can you get by with two pairs of comfortable shoes? That’s probably all you need, as one may need a day to dry out if they get wet. Consider any item that requires electricity. Do you really need to take it? If so, an adapter will be needed. Limit the toiletry items to the basics. Either pack travel sized items or plan to pick a few up while there.