i sent a letter on wednesday which hasn't got to its destination as yet, it had a first class stamp on it, the 12 stamp pack were from a few months ago, seeing as how the price has gone up i think, would it still be valid.
As long as the stamp has 1st on it, it will always be valid as a first class stamp.
Some people buy loads just before a price increase to save money because they know they will still be valid for years.