Are Pounds Sterling referred to as squid (in addition to quid)

This is quite common in the North East, but only among the common (or, let's say the 'down-to-earth', or street-smart, whichever you prefer) - particularly the youth. There are others, too, such as:

• bin lid
• nicker
• cherry

Money-slang also extends to multiples:

• score = £20
• pony = £25
• ton = £100
• monkey = £500

And so on; I'd put my money on what you saw being intentional, rather than accidental.

I've heard it occasionally as a joke. I don't think it is (currently) any more than that - though it might become so, in the way that "guesstimate" seems to be overtaking "estimate".

I don't consider it common, but it's certainly becoming more so. Robert Peston, Sunday City Editor of the Daily Telegraph, has used it in print at least a couple of times:

...various bits of a broken-up plane that could be yours or mine for just a few thousand squid. (source)

If there is a few thousand squid to be reclaimed, I should be able to remember that no one at the bank ever warned me that market interest rates might actually fall. (source)

I've always thought this was a reference to jokes based on 'sick squid' vs 'six quid' as in 'Here's that sick squid I owe yer' etc