There are 56 countries and territories that do not need an entry visa for stays of up to 6 months in the U.K. Only 52 countries are allowed to enter Thailand under the visa waiver rule. So you could actually say the UK is more lenient than Thailand on visa issues. The UK, like Thailand requires many countries to obtain a visa beforehand because many decide not to leave.
Again, it is subjective. Different embassy officers may see things differently. When dealing with claims it may come down to the individual officers subjective reasoning. i am not defending it, just saying it is a difficult subject and not cut and dried. I do not know of any of the many people I know here personally who have been rejected.
For every story like this, i can give you 5 where people got the visa and did not use an agency. This is over the 36 years I have been here. The one major criteria the UK uses is evidence that the person will return home at the end of the visa period. In the cases you mention i can only guess that they saw something that led them to believe differently.