The Acorn BBC model A is the successor of the Acorn Atom and its first
name was Acorn Proton.
It was a very popular computer in UK and was widely used in school,
but it didn't have a great success elsewhere (even if it has great
characteristics, it was too expensive). 

The Model A doesn't have some of the connectors of the Model B/B+
(User port, Tube, ...) on the underside. 

This computer got it's name because, in 1980, the BBC decided to start a
computer literacy television series.
The network realized that, with more powerful and increasingly inexpensive
microcomputers, it would soon be possible to create them with enough
computing power to offer their owners personal hands-on experience with
microcomputers at an affordable price. 
The BBC considered the NewBrain computer and rejected it.
Acorn and Sinclair Research, along with other companies, then submitted
designs, and the Acorn won.
The BBC model B was then almost universally used in British schools from
it's birth into the 90's. 

It was followed in 1982 by the Acorn BBC model B, the Model B had the same
characteristics but had 32 KB RAM (up to 64k). 

The "Tube" is an expansion port which was designed to connect other
processors to the BBC (6502, Z80, 68000 or ARM 1 RISC).
An interface was designed for the tube and used an other 6502 and a Z80. 

One of its most popular extension was the "Torch" floppy disk unit,
it was a 5.25" floppy disk drive with a Z80 which allowed the BBC to
use CP/M softwares. 

Acorn made a "cheap" version of the BBC (fewer connectors & video modes)
called Acorn Electron. 

In 1985, the Acorn BBC Model B+ is released.
The Model B+ has new features : 64k of RAM instead of 32 KB, it has
internal circuitry for the Econet and Disk Drive as standard, they were
both available as an upgrade in the models A and B.
The later models included disk support as standard, using either and 
8271 or 1770 disk controller. 

It was followed by the BBC Master. 

(www.-old-computers.com)