use old iphone 6s for travel?

As long as the phone is not still locked to a carrier you should be able to get a short-term SIM card for it. I presume the UK has them but you would have to check. I know some countries such as Japan do not allow this.

From: How to unlock your iPhone for use with a different carrier - Apple Support

"You can find out if your iPhone is locked by going to Settings > General > About. If "No SIM restrictions" appears next to Carrier Lock, your iPhone is unlocked"

Prob will use WhatsApp for calls to other countries, but can't always count on good wifi.

Use Public Wi-Fi -> see this Apple article: Use captive Wi-Fi networks on your iPhone, iPad or iPod touch

Use captive Wi-Fi networks on your iPhone or iPad – Apple Support (UK)

Nowadays, Public Wi-Fi is offered for free in many countries, in big cities, shopping centers, in Public Transport (bus & train), airport, restaurants, hotels and others.

It should work. I had an older model (5c) I took to Germany 5 years ago and after buying a SIM I had phone service for 30 days (plus limited text and data) with an Orange SIM. If you are planning on calling to another country make sure the plan covers you for this and about any extra charges.

Oops...

If the iPhone 6S is carrier unlock iPhone - SIM free, then yes if you use a physical SIM card, also a prepaid SIM card.