Lex, see my March 20 post regarding Cilantro. It should grow like mad in the UK, given rich soil, some fertilizer, but most importantly: sow seeds in late summer-fall, not spring.
A weak-tasting Oregano is likely “Pot Marjoram”, not true Oregano. Technically, they’re all in the genus Origanum, but Greek or Italian ( Origanum vulgare var. hirtum) will be better flavored, stronger. Those varieties are fuzzier and require less water-try to keep the foliage as dry as possible when watering.
Basil requires warmth, sun, lots of Nitrogen fertilizer and pinching off flower buds before they set seed. Eventually, Sweet Basil (Ocimum basilicum) , and others will need replanting when quality goes down. The miniature-leaved one (Ocimum minimum also O. basilicum minimum) are originally from Greece. A good friend helped introduce it into the USA decades ago.
Unfortunately, Basil Downy Mildew was recently introduced into the USA via contaminated African Basil. This disease forms dark grey patches under and on the leaves. It kills plants rapidly and spreads like fire. “Resistant varieties” don’t truly exist. Eleonara is a cultivar with some resistance, but not much. Infected plants and leaves should be bagged up in plastic and thrown away as soon as the disease is noticed. Within days of infection, all the leaves will yellow and fall off.