Summary
If you have a lot of painting experience, and you’re working at a professional/self-promotional level, I’d suggest you aim for the “artist-grade” rather than student-grade. Here’s a tiered approach:
Main working range
Choose one brand with strong pigment load, reliable quality and good availability in the UK. For example Michael Harding or Winsor & Newton Artists’ Oil.
Experiment/secondary brand
Pick a second brand (e.g., Jackson’s or Gamblin) for special pigments, different texture, or cost management.
Large scale / cost-sensitive pieces
For preliminary works, studies, or large canvases you may offer at lower price points, you could use a slightly more budget-friendly artist-grade brand (or even student grade if the final market tolerates it) but keep the core work on premium paint.
Colour & pigment strategy
Since you’re selling and showing work, pigment quality matters (especially for lightfastness). Also consider how the texture and behaviour of the paint suit your style (do you do heavy impasto or thin layers, glazing vs direct painting?).
Budget & value
Premium brands cost more — but reviews show you do get more pigment, better permanence, more consistent behaviour. One article: “you really can be guided by price to gauge quality”. Lin Kerr – Artist
Availability & supply
Since you’re UK-based, double-check local stock, shipping, tax, etc. Brands like Winsor & Newton, Jackson’s are UK friendly.