They are Machins. The name derives from the sculptor Arnold Machin who designed them in 1967. They have been in use for general franking purposes since 1967.
They are so-called definitive stamps: stamps intended for franking over a long period of time. They do not commemorate or celebrate anything. Hence, there is no year on them. - Although I do not understand why a stamp should have a year on it. - Unlike the definitives of most countries that are replaced by a new set every five years, the UK has used them for almost 54 years, now.
As tariffs changed over time, new values were added to the series. Technologies changed, different printers were contracted and even a monetary system changed during the lifetime of these stamps. They also come with regional emblems issued in regions of the UK. There are thousands.
For a good listing:http://connoisseurcatalogue.net/catalogue.htm
If you find a 4d (pre-decimal) sepia stamp with 2 phosphor bars, Arabic gum and a B2-type head: £ 1,500
A 3 1/2p with olive-brown instead of grey-green colour mint sets you back £ 850
Or a £2 with the £-symbol missing will set you back around £ 150.