Yes Britain has certainly changed but not for the better I'm afraid, but it is not only down to immigration.
Yes mass immigration has caused the largest change, far too much over such a fairly short period of time.
The wrong type of immigrant, ie.bringing in with them their own culture and diverse life styles, that in most cases doesn't fit in with our own ancient culture.
This is not being racist, I and many others do not care either for the British dependant on benefits culture, who adopt a slovenly life style, and refuse to work. Or the travelling communities that pay no taxes, and leave areas trashed and rubbish filled when they move on.
One can say another cause of change, as been an abused Benefits System.
Another cause of change has been our entry into Europe, but that is a problem all of it's own.