Britain's diminished role as a nuclear power, a permanent member of the UN Security Council, a provider of support when natural disasters strike, a political and military ally to the US, a member of NATO and a keeper of international promises.
You may not like the role that the UK plays on the world stage and it's certainly not as dominant as the late 19th century but the UK has a very strong international reputation and a recognised military capability.
If you want an example of why we need a strong military consider Hong Kong. We can't send troops in there to protect its citizens against Chinese oppression but we can project force into the South China Sea to protect the interests of other countries there, and we must protect ourselves against Chinese retaliation for upsetting them through our diplomatic work in both locations, along with any military involvement in support of (e.g.) India, Malaysia or Taiwan.