We'll be visiting the UK and Ireland in August. Are there any prepaid SIM cards that offer reasonably priced data in both countries or should I plan on buying SIM cards in each country?

From what I've seen so far, it seems like a SIM card in each country is what will be required.

I was thinking of the £10 O2 500MB, 100 mins, 1000 texts plan for the UK, and something on "Three" for Ireland.