I’ve used Car Parts 4 Less, Euro and GSF without too much trouble. Euro and GSF are best when they have a sale on, which as has been said is most weekends at GSF and always for Euro. I believe Euro now price match too.
However I have just serviced my car with some items I bought from Trade Parts Specialists (TPS). They are part of the VAG group, and only sell genuine VAG items. They’re supposed to be trade but were willing to sell to me, and I got some discount, although not full on trade prices.
5 litres of Quantum Platinum oil 5W40 (VWs own brand fully synthetic oil made by Castrol) was about £25. I also got a pollen filter, air filter, oil filter, sump plug washer and a set of plugs for just over £36 in VAT. Comparable to Euro and GSF prices so worth considering for VAG group cars.