Clients mother has a UK Pension and USA Social Security

a citizen of the USA worked in London for a few years. She has a UK pension of 172.48 English pounds per week. They are offering her a lump sum payment of 52, 358 English pounds. How is that taxed both in the USA and in the UK. She has never filed a tax return in the UK and has not included this money in her monthly income in the USA. She does get Social Security in the USA. 

Based on my research, it looks like she should have been filing this with the USA for the last 10 years and has not. I am surprised that Britain does not take a mandatory tax from such benefits for non-citizens and non-residents. Has that always been true?

-Bill S.