The principles of roaming are the same whether you're pay-monthly or PAYG, but the costs will differ.
WiFi-calling is a coverage-enhancement designed for use in the UK, and not a means to reduce or avoid call-charges. This applies equally abroad, where WiFi-calling is not guaranteed to work but will incur the exact same call charges as if you'd dialled over the mobile network. WiFi-calling needs to be differentiated from apps that enable an in-app voice session between users, whereas WiFi-calling means making & receiving calls entirely as normal without an app.
The roaming pass you mention, enables full use of your UK allowances for £25/week - which means outgoing calls to both the USA & UK, incoming calls, and access to your UK data allowance (with a 50Gb/month FUP) are all inclusive. Usage of internet-based apps - be it Sky Sports, BBC News, WhatsApp or any other - is classed as data usage (be it over WiFi or mobile-data) exactly the same as anywhere else.
A local-SIM (whether "e" or "p") may be cheaper for purely in-country usage and for data, but will come with a USA number which will be international rates for any of your UK contacts to use. With the roaming pass, they will contact your UK number as usual, with the incoming costs covered.
Whether the convenience offered by the roaming pass is worth the expense compared with your trip, is something only you can decide - whatever you choose you must make sure the free roaming facility is enabled on your SIM. This unlocks access to foreign networks and ensures you have coverage abroad. Text ROAMING to 150 before leaving the UK.
