While we have to rely on estimates of GDP prior to WW2, like most countries the UK has seen steady growth in GDP since the beginning of industrialisation in the late 18th and early 19th century. By that measure the UK is (by far) richer now than it has ever been in the past. Source (Spreadsheet download warning)

The UK is, of course, now a much smaller share of world GDP than it was a century ago. Again, although numbers are somewhat hazy the UK accounted for around 10% of world GDP in 1870, whereas they only account for around 3.5% today.