Hi @acaley! Welcome to The Netherlands and welcome to our forum! In this situation I'd advise you to first get your mobile subscription sorted. You can use your UK ID for that, however then your only option is to go to a KPN store to order your subscription. They’re the only ones that can handle non-Dutch ID papers. Unfortunately, due to the COVID situation, many stores are closed now. We do have some KPN XL stores that are open. On this page you can find the store closest to you (for Utrecht it's the one in Nieuwegein) and you can also make an appointment online for when you go to the store (this is obligatory, also due to COVID). The site is in Dutch, but if you put the URL in Google Translate, translate it into English and then click the URL in the English box, Google will translate our website for you. 

If you'd prefer to wait with ordering a cellphone subscription because you don't feel comfortable travelling to a store right now, you can also order a prepaid card. You can pick the €4,99 one. At least then you'll have a Dutch number and that enables you to order the internet services for your home and the card should be with you within two to three business days (provided our postal services have caught up after the holidays). 

Once you feel safe travelling to the store again, you can order a subscription and ask whether you can keep your prepaid number (nummerbehoud) should you want to. Don't forget to bring a proof of registration (BRP) to the store as well when you go! Here you'll find how to apply for one in Utrecht.

Lots of information, but I hope it's helpful to you. Should you have any other questions, do let me know. Happy holidays for now!