Yeah if you think about it mathematically, everyone has 2 parents, 4 grandparents, 8 great grandparents and so on. So you dont have to go very far back in generations before you far exceed the population of the UK:
2
4
8
16
32
64
128
256
512
1,024
2,048
4,096
8,192
16,384
32,768
65,536
131,072
262,144
524,288
1,048,576
2,097,152
4,194,304
8,388,608
16,777,216
33,554,432
Thats 25 generations. Lets assume each generation is about 20 years (being the average age at which any one generation gave birth to children). On that premise you only get back 500 years from the above.
The conclusion is that we are all, to a greater or lesser extent, inbreds