If it's a matter of UK citizens living abroad, the stats are that there are nearly 5 million Brits living overseas, and less than 200,000 were registered as overseas voters at the 2024 election. So there's a difference in magnitude (and I don't think there's a big push on to get them all registered)
British overseas voters can vote in Parliamentary elections, but not in local elections. Which is fine. If you're a UK citizen abroad, you've got an investment in who forms the next UK government, but you're probably not going to be affected by who sits on Tintagel Borough Council.
And there's also the question of whether citizenship actually means anything, if there is a nation, or if you take the Green Party view that anyone who happens to be in the country should have exactly the same rights and entitlements as a citizen.
As far as Commonwealth voting goes, like I say I'm not totally opposed to it. There is a political issue where normie white Brits don't know there's such a thing as Commonwealth voting and are shocked to learn that foreign nationals can vote and indeed be elected. They'd be even more shocked to learn that there's no requirement to have a long-term right to remain in the UK, all you really need is a residential address.
I think a lot of this could be defused by having a residency requirement. Most voters will apply common sense and say that obviously the Jamaican nurse or Pakistani grocer who have been here for 40 years are a part of their community. The Deliveroo driver who just recently came here on a questionable student visa, not quite so much.
Another thing few people have noticed is that parliamentary boundaries are drawn up based on the size of the electorate in a particular area. If you get automatic registration of potential Commonwealth voters, that doesn't just add a couple of million voters to the register, it means more seats in London, Birmingham and Manchester. There are cynics out there who might think Labour is playing silly buggers with the electoral system to try and give itself whatever marginal advantage it can think of.