Hi Tweet
Yes, you can buy alcohol at Heathrow and it's probably a good idea to do so - I remember paying the equivalent of £6 for a pint of beer in Oslo! I believe you can buy up to 1 litre of spirits or 2 litres of wine at Heathrow, but you can check this when you buy. Also, Norway is not a member of the EU, so you will be able to take advantage of duty-free prices.
Have a great trip - it's a fascinating country.
Kind regards
Olly, your editor