Speedking...Yes, there are lots of makes of bike -
Decathlon are relatively new to the UK and have I think 3 roadbikes in their range and about 10 stores dotted around the UK. Specialized have dozens of dealers for example and
Halfords have a branch in every town almost. They also sell a lot of bikes through Cycle to work schemes - I expect that's partly why you see a lot in London
The Triban 3 is very well reviewed and offers very good vfm for £300. You can of course get a better bike for double or triple the money but a not insubstantial part of that cost will be paying for branding and marketing which Decathlon don't do too much of, relying on their good vfm and quality for the price
Pretty much all of the frames are made in the same factories in the far east, it is the quality and weight of the components (and the marketing) that determines the overall cost
You pays your money and takes your choice - but the £300 Decathlon is probably not far off a £500 Specialized, Giant, Trek etc...
I certainly wouldn't discount a bike on the basis of the name
I have a Pearson - heard of them or seen many on the road?
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They have had a shop since 1860...