As a follow-up to my previous discussion, I though I'd record my results here for posterity/Google.
I was trying to match Mathmos' violet liquid, and was having trouble since most dye recommendations were only easily available in the US. I found some UK sources for food dyes, and while I didn't quite succeed, I'm happy with the results.
The two sources are Rainbow Dust (their Colour-Flo range is a solution; avoid their Paint It! range, it's a suspension) available from Amazon, and Squires Kitchen (their Professional Food Colours range), available direct.
Finally, here's how I found a rather nice purple. It's not as vibrant as Mathmos' violet, but it's as close as I could get without wasting more money on experimenting.
First, I tried Squires' Violet and Rainbow's Purple, but both of these tended towards maroon when diluted. Eventually, I settled on :
4 drops Rainbow Dust Colour-Flo Red
1 drop Squires Kitchen Wisteria
1 drop water
1 drop of this mixture per 500ml Mathmos original liquid (mine had faded to completely clear).
This gave decent results that stayed purple when diluted. If I had more money to waste, I'd try a lighter red, or even pink, to pull the purple up into violet.
Hope this helps anyone looking to rejuvenate their UK lamp!