I'm not convinced of Uberuce's wine analogy.
Sometimes all you need is a simple bike with the features you need. The difference between a £500 bike and a £300 one is sometimes not that huge: it depends on the brand, the spec, what extras it has.
It is nice to have a higher quality bike, but not always essential. There are quite a few bikes in the price range Marianne is thinking about that could be suitable for her needs. Personally I wouldn't go cheaper than around the £300 mark unless buying second hand.
Some of the bikes at Decathlon seem very good value: The 'Elops 7' offers hub gears*, hub dynamo, lights, mudguards, lock, rack... Most bike shops in the UK will not offer these features on a bike at this price level. It's a frustrating characteristic of the market here, which is mostly geared to leisure/sports cycling. The only questionmark with Decathlon is after sales service/maintenance, the kind of issues you find at Halfords perhaps.
The similar Raleigh Caprice is more fashion oriented in styling, with retro painted steel mudguards, front basket and swept back shopper bars. No dynamo, but hub gears for ease of use/maintenance.
*- There's a 24 speed derailleur geared hybrid too, the NeWork 5, also with dynamo lights, rack and guards.