If you want an iPhone to use sim only, it's probably better to get it direct from Apple. You will have more choice of storage sizes and also colours to.
The same can be said about the Apple Watch too (well more colour and case material choices).
The cellular add on for the Apple Watch is around £7 per month (this is only needed if you want to use the AW when out of bluetooth range of the iPhone). If you will always have your iPhone with you, then there is no use in getting the cellular add on.
Here's a selection of the sim only tariffs EE do, some of which are 5G:
https://shop.ee.co.uk/sim-only/pay-monthly-phones?search=%3A%3AsimoServicePlanDataSpeed%3A5G&expandF...
