A loose Perfect Order pack at around £4.50 RRP is the simplest Pokémon TCG transaction there is. Ten cards, one rarity tier or better guaranteed, and a chance — slim but real — at one of the set's Mega ex Special Illustration Rares.
Perfect Order single packs are most useful for collectors topping up specific gaps in their sets. The full master set is achievable through volume opening, but most UK collectors who chase SIRs end up combining loose-pack purchases with secondary-market singles for the cards they don't pull. The loose pack route works because it keeps the opening experience open-ended without the financial commitment of a Bundle or ETB.
Specialists carry these — Chaos Cards, Total Cards, and Magic Madhouse all keep Perfect Order packs in their listings — and prices are usually steady at £4.50 or just under. Smyths and Argos are less reliable for single packs but stock them inconsistently. If you're buying in volume, specialists will sell sealed cases of 36 packs (effectively a Booster Box's contents in pack form) at slightly better per-pack rates.
For competitive players, single packs are the wrong product. Buy the singles you actually need from the secondary market — it's almost always cheaper. For collectors and casual openers, loose packs are exactly the right scale.