If the stamps are maked 1st or 2nd then these are accepted even if the postal rates have gone up in the meantime.
If the stamps are marked with a value, then you will have to add extra stamps to the value of any difference between that and the current postal rates.
All stamps issued since decimalisation (?1972) remain valid.
Pre-decimalisation stamps are valid if they are for �1, �2, etc - ie no shillings or pence. Not sure how far back this goes - but anything really old will be worth more than its face value to a collector so it would be silly to use it.
Yes, you can if they're marked Second Class, rather with a specific financial cost on them, i.e. 28p, etc. but make sure the Christmas card envelopes are below the new size for which a higher rate postage is now applicable.