Interesting that people here think that $5 roaming is a good deal. In the past when I've gone to the UK I've just bought a local Vodafone pay-as-you-go monthly plan and paid £20 (about NZ$39 at today's rates) for 10GB and unlimited UK calls and texts. If I instead used roaming for the 1 month I was away then it would cost me $150. The only advantage to using roaming would be that I could receive calls to my usual number, but that's not of much use to me as I rarely receive calls and the few people that are likely to call me would know that I was away anyway.
Or am I missing something?
As others have pointed out, VF global roaming is not the best option for 1 month on holiday in the UK (or likely, any other single country).
But you are missing something else in your calculation for people on Prepay. There are two VF Prepay global roaming options - $5 /day as you mention, and a $19/seven days option. That works out at $76 for your 30 day trip, which is half your $150 estimate. Not difficult to make four 7 day packs work for 30 days since the 7 day option covers the first part part day (whether it is 23 hours or one hour), plus the following seven full days.
It is ideal for the type of business travel I do; eg. up to five countries in 2 weeks (or even 4 in ~one week at times). Just need to make sure to renew the package as soon as the first one runs out (or rather turn roaming off just before the pack runs out, wait until the new day has rolled over in NZ, buy a new 7 day pack and turn roaming back on. Not hard to do once a week). Otherwise pretty much set & forget. We also find it good for holiday use, even for 2 weeks in one country. Beats fluffing around using local sim cards even if they do work out a bit cheaper (which will not necessarily be the case). But the big plus is multi-country. The one pack works seamlessly across borders; eg. Australia - Singapore - UK - Bermuda all on a single 7 day pack
Yes, there are many countries not covered but it does cover the large majority I have travelled to or through (main exception for me being UAE, and that just in transit for a few hours).
The other exception may be if you really do need the full 10 GB/month or have high call volumes. But the 7 day pack gives 1 GB data, plus 100 mins & 100 texts for the week; ie 4 GB for the month). That will be a limitation for some but with hotel or office wifi available, works well for me.