Beatties was founded by Colonel S N Beattie and Charles Whale as the Southgate Model Shop. In the early 1960s they had two small shops on Winchmore Hill Road in London. Looking away from the tube station the one on the right sold new model railway equipment and the one on the left was full of used stuff. He later bought out the Bassett Lowke shops after Whale left the company.

I think the Holborn shop (which I too remember well) was originally a Bassett Lowke one. 

Bassett-Lowke's decline, starting in the late 1950s, can be blamed on at least two factors: people would browse the firm's free catalogue and buy similar or nearly identical items elsewhere at lower price; and the interest in technical toys declined in the late 1950s and even more in the 1960s. ... In 1964, the company ceased retail sales and sold its shops, including one at High Holborn in London, to Beatties. Bassett-Lowke went out of business in 1965.