Introduction: 

After the Second World War the British army needed to find a new rifle cartridge. They used the rimmed .303 British for decades and it was obsolete by now and had to by replaced. 
During the Second World War the German army had developed an intermedium cartridge (the 7.9x33mm kurz) for their assault rifle the mp43. After the Second World War the UK investigated the technical development of this captured rifle and its cartridge. 
The advantages of a short cartridge were the amount of rounds an individual soldier could carry and the increased firepower. (It is easier to manipulate the rifle when fired fully automatic.) 
The ideal caliber panel conducted a theoretical study to find the ideal caliber and they came up with 2 possibilities; a .250 with tungsten carbide core or a.276 round if tungsten carbide cores were forbidden. 
 
With this information the British army designed two new types of ammunition, a .270 round and a .276 round. 
The .276 which was later changed in .280 (for cosmetic reasons), was the best of these and it was developed in a wide range of loadings. 
The .280 was shortly approved for service in the early fifties as 7 mm MK 1 (together with a new rifle the EM 2), but finally it was replaced by the American .30 T65 E3 (later known as the 7.62x51 NATO round). 

The .280 cartridge case:

The .280 cartridge has a rimless necked case and it is made of brass. The length of the case is 43mm and it is berdan primed. The total length of the whole cartridge is 64.3 mm. 
In 1949 the head of the .280 case was changed into .280/30 to fit in rechambered American rifles because the head had the same size as the head of a .30-06 and a .30-T65 cartridge.

.280 en .280/30 

The .280 cartridge case was produced in brass but there were also some experimental cartridges produced in aluminium, lacquered yellow, blue, plane or orange.

The bullets:

The first ball bullets weighted 130 grains. 
There were bullets with lead cores and bullets with mild steel cores, both called type A. 
The mild steel core was identified by a dark blue bullet tip, the lead cored bullet by a plane bullet tip. 
The later ball bullets weighted 140 grains these bullets were both called type B. 
These were also produced with lead and mild steel cores, identified by a yellow and a green tip. 
Other ball bullets were further developed with minor changes of the mild steel core ( for example type C with the pink bullet tip and the type AA with the chocolate brown bullet tip.)

                                                                                                   x-ray of 140 and 130 grains ball (yellow & blue tip)

In 1949 the UK started a cooperation with the Belgium factory FN and the Belgium engineers suggested to use the 7mm Mauser bullet (type S12). This bullet has a CNCS envelope, a lead core and weighted 140 grains. 
The ballistics of this bullet was very good and the British introduced this cartridge as the 7mm Mk 1. Some of these 7mm Mk 1 bullet had a yellow tip but because of the CNCS envelope it cannot be confused with the British lead core type B bullet. 
In 1970 the UK produced a 7mm Mk 1 with a GM envelope and a purple tip. 

 Other types: 

The .280 and .280/30 is also produced in AP, tracer, API, observation, proof and grenade firing cartridges. 
The AP round was identified by a eggshell blue bullet tip, the tracer round by the white tip color, the API cartridge had a black bullet tip and the observation was marked with a red tip.

A drill round was produced by using a cartridgecase without a primer, filled with coaldust and leadballs (for the weight). It was sealed at the bottom with a carton disk. (see image)

The Grenade cartridge was made of a rose crimped case. The FN produced grenade cartridge was slightly different. 

The proof round is identified by one or two red painted bands on the base of the case. Very little (inert) copperwashed proof cartridges were produced for illustration purposes.

Here are some images of my own collection including other 7mm rounds as well...