ACG is second to nobody, serious basses from Scotland.
ACG are amazing, and there are loads of brilliant small boutique bass builders in the UK. We're probably better known for boutique basses and acoustics than we are for electric guitars, these days!
The reason that there are no real major UK-based guitar brands now is probably due to the fact that the revolution of the instrument began in America and the biggest bands here in the 60's (like the Shadows, the Stones and the Beatles) started off using some dodgy European gear and quickly moved to American instruments when they could. Everyone remembers Bruce and Hank playing their red Stratocasters and the dodgy Burns guitars they used to play were consigned to the dustbin of history.
This also needs to be considered in the context of UK manufacturing history. UK manufacturing practically collapsed in the mid 70's and never recovered. The country was very badly mismanaged by successive Labour governments at the time and we had some of our worst economic years since the end of WW2. The Labour-backed unions became too powerful and held too much sway with the Government and they effectively put a stop to many industries as and when they pleased. It was only when the Conservatives came back in the late 70's and early 80's that the country was dragged back from the brink of the abyssal basket-case it had become. British manufacturing never recovered from this, the damage already having been done, so the UK economy turned it's sails towards finance and services. The British guitar brands were all sold off or moved production overseas and that was effectively the end of them.
Unlike America, the pride in British manufacturing died in the 70's and never returned. There was never any value placed in anything that was made in the UK. Consider how our car industry became a laughing stock and collapsed (though Rover soldiered on bravely until the millenium and Jaguar Land Rover managed to survive by luck more than skill). We openly mock British-made products like Vauxhall, yet froth over supposedly superior German products. If you tried to argue that a Honda made in Swindon was an excellent car, someone would tell you that it was only good because it was Japanese and that the factory runs the famed (Toyota-born) Japanese production cycle. UK manufacturing is not financially competitive on any sort of scale, and customers here just do not appreciate quality enough to pay more for it.
However, the UK continues to be a centre for innovation with some excellent craftsmen, hence why we build such great boutique basses.