Are two regular sized first class stamps enough to send a A4 sized letter.?

Really what you want is a 'Large' stamp for the correct weight, but as long as it's under 250g (£1.26), two conventional 63p 1st class stamps should work.

This is a "large letter", and how much does it weigh? If it is up to 100g, the price is 95p, so two first class stamps will be enough but you're spending more than you need. For 101g-250g, the price is £1.26 so two first class stamps (which currently cost 63p) will be exactly right. There ARE other stamps available and that's what post offices are for!

Where it's going is irrelevant if it's inside the UK.

Weight matters as will the shipping options.
There is no all-purpose stamp.

Almost certainly if they are A4 sheets. Be sure to squash it down to as thin as possible too.

I should think so, yes. But to be on the safe side, why don't you ask at your local post office?

No - you need to take it to the Post Office to make sure it fits through the magic slot they use for working out how much postage something needs. Also for an A4 letter envelope you need the large letter stamps.