Have you tried Bourjois Smoky Stories Eyeshadow Palette Grey & Night. One of the grey colours may be similar to your discontinued favourite. It is so annoying when you find something you love, and they stop selling it in the UK.
In case anyone's wondering about the Rimmel ScandalEyes Liquid Eyeliner, it's actually a pen, not a brush (my favourite way to apply liquid eyeliner). Possibly similar Soap & Glory's Supercat, which I quite like.
I did stockpile Constance Carroll/Collection 2000 loose powder from an ebayer who was selling in bulk years ago. Suppose watching job lots/bulk listings on ebay is your best bet for discontinued makeup. My most recent bulk buy was Neutrogena Facial Cleansing Bar (imported), which I ruddy love but hasn't been on sale in the UK for years. Neutrogena Clarifying Shampoo is fantastic too (another one we can't purchase in the UK any more).
I know what you mean about cosmetics "disappearing" from sale. I've used Bourgeois little pot eyeshadow in Argent for years. You can still get most colours on Ebay for decent price (around £3-£4),but Argent (shiny mid-grey) is a ridiculous price like £12, if at all! (confused) How on earth does that happen? Been monitoring situation for ages (annoyed)
Also makeup related, but not Poundland - this is probably the best price I've seen for this in a while - we get ripped off for Coty Airspun as it's not sold in the UK. £8.68 + Free Delivery, Coty Airspun Loose Powder in Translucent (huge 65g pot, so lasts for ages). Suits my pale skin fine, only downside is its fragrance, which isn't great). https://www.amazon.co.uk/COTY-Airspun-Loose-Face-Powder/dp/B000BNG4VU/ Rosey Beige is £7.21 + Free Delivery! https://www.amazon.co.uk/COTY-Airspun-Loose-Face-Powder/dp/B003LQIUPC Naturally Neutral is £8.41 + Free Delivery (this shade actually looks lighter than Translucent! may give this a go) https://www.amazon.co.uk/gp/offer-listing/B000052Z5B/ref=dp_olp_new?ie=UTF8&condition=new I had a bit of a grumble about loose powder a few days ago.