Can one family iphone be used as a hotspot for other iphones?

I can't see why not. Once one phone is transmitting a wi-fi signal anybody in range can get it. It will be limited by the data speed of the transmitting phone. So if 4 members of the family decide to stream movies, could require a referee. And if the plan has a data limit, could max out in a hurry.

Yes, you can do this - I have with my Android phone. But keep in mind a phone's WiFI hotspot may not be designed to be handle five connected devices at full speed, the way a WiFi router would. Don't expect everyone to be able to stream movies at the same time or it could be super slow.

Also, make sure the mobile plan and SIM you are purchasing allows the use of hotspot. Some do not. ASK when buying in a store, and if possible try it out with another phone before you leave the store. There are several different companies offering SIM cards in the UK; if one doesn't allow it, try another:

http://prepaid-data-sim-card.wikia.com/wiki/United_Kingdom

"Tethering" is another word for "hotspot."

Andrew beat me to it - the most important thing to check is if the mobile plan you are buying allows tethering, how much, and at what speeds.

For instance, my mother's T-Mobile plan in the US allows 7GB per month of 4GLTE tethering; my T-Mobile plan allows unlimited tethering, but only at 3G speeds. And just as in the US, the terms and options change all the time, and there are many different plans available.

I bought a Vodafone SIM with a big value bundle plan last year that allows tethering. IIRC it was £20 for 5GB some minutes and texts too. I think the minutes and texts were to U.K. numbers only. It was a little tricky to get the hotspot feature working and I had to get help at the Vodafone shop. There are cheaper options but I went with Vodafone as they seem to have more 4G roaming in Europe.

https://www.vodafone.co.uk/mobile/pay-as-you-go-plans/big-value-bundles

My current provider, Three, used to allow hotspot tethering but now they've stopped supporting it. I've tried it since the changes and noticed considerable throttling so whilst my phone is capable of supporting it my provider doesn't want me to.

I can't speak for other providers but I suspect Three would not be alone in this regard.

Complimentary wifi is so ubiquitous in England that there isn't really any requirement for everyone to have a UK sim or use a hotspot.

Thank you all for the very helpful information. I will have my husband - who is more tech savvy than me - read your responses also, so we can figure out what option might be best. We are staying four weeks, and our teenagers fare much better on the long train and bus rides if we have wifi available for them. Last time we did this trip, the wifi provided on the buses and trains was hit and miss. We mostly stayed on airplane mode for all our phones unless we could get wifi. I think one of these options you all discussed will surely work for us. Thanks again!

The wifi on trains and buses will be hit or miss now too. Mobile wifi depends on the train's or bus's router being able to have good cell signal from nearby cell towers. In very rural areas - where trains and buses may go - there won't often be a good cell tower in the right place.

Then the hardware has to be really good at handing off between cells. If it is second rate or getting old that could be a problem.

Then, wifi disappears in tunnels and cuttings (where the surrounding countryside has come up around the road or track, but not covering it.

Unfortunately you will still have most of these problems when trying to use a hotspot or tethering in those areas. It won't likely be better than you will get with the train wifi.

I don't use the wifi provided by my train company. I prefer to muddle through with my 4G on my phone, but I work for the company so I know where all the dead areas are on the lines that I work on. Still frustrating though...

I think it would be a good idea for your teenagers do go without their mobile phones and see what like would have been like in the not too distant past.