Race and culture are NOT intrinsically linked.
However, there is usually a very close correlation between the two given the culture is normally that of people with the racial physical attributes from where that culture is from.
What the OP is asking is why there appears to be a cultural sub-group of people in the UK who seem to shop in the way he has seen (for the record observational evidence is VERY much an accepted form of evidence in certain sciences, albeit the OPs recording methods may not stand up to scrutiny

). The fact that they are racially from the Asian region (the OP doesn't specify) merely allows us to pin-point the most likely cultural sub-groups the people come from.
What the OP asked was not inherently racist, however i cannot rule that out either as I simply do not know the OP.
My own personal experience of retail work (Homebase so large DIY store), was that there was a very certain type of person who came in a few minutes before store closing, in my case it was nearly always somebody with an Irish accent who emerged from a battered Transit van. They would then wander round for a few minutes after closing and then ask to see the manager to discuss a bulk discount on some such thing or other.
In the 5 years I worked there part time I probably saw this happen maybe 10 or 15 times, always a different person, always fitting the criteria.
Ultimately though, I didn't care enough about the job to care about why these people did what they did, all I knew was that they didn't pay over-time and I wanted to go home
