What exactly does this mean?

Yes that’s what it means.

Did you review all their cancellation options? Flex, Semi Flex, Advanced et al?

OK, thank you!
I'm trying to wade my way through all the fine print as we speak.

I don’t normally do this, but last summer I payed extra to have more cancellation flexibility in all the places I stayed in Wales. One of them was the Premier Inn.

Why did I do that? Because even though I had both Moderna Covid-19 vaccinations and both boosters I knew that the variants present at the time could still get me. I also knew that there were virtually no requirements in GB for masking, sequestration, etc. As it turned out, I did get it, probably in Northern Ireland. Fortunately, the only symptom I had was a slight fever for one day that kept me in my B&B room in Llandudno and I didn't have to cancel any hotel reservations anywhere.

I was actually sicker from what turned out to be an allergy to the blooming Jacaranda trees in Portugal and Spain in 2019. That didn’t make me have to cancel any hotel reservations, but it did keep me from going to some self-scheduled activities as well as to some on the 2 RS tours.

The point is that anything can happen and unless you have travel insurance to cover having to cancel or pay extra if required for flexibility, canceling could be pretty costly. The Premier Inn in Aberystwyth charged very little extra for the flexibility I might have needed and the cost was well worth that minimal price.

Lo, I'm sorry to hear you became ill on your trip. When I was in England this past spring, my husband, here at home in the U.S., got covid for the second time. I was fine and didn't get sick.
I do have travel insurance which has a covid clause in it. I have just about everything except "cancel for any reason."
I don't remember there being these "flex" and "refundable" options before the pandemic. Were they around and I just never noticed?

Essentially by day 28 it becomes a non refundable accommodation. Unless you absolutely must stay at this establishment I would look at other places that have a more generous cancellation period. Another option would be to wait as late as you feel comfortable before committing to your preferred location.

Yes the flex and refundable options for hotels were around long before Covid as well as the pay now. After getting sidetracked by something completely out of my control on a non refundable many many years ago I’ve avoided them. It was neither the merchant’s fault nor mine.

Premier Inn added the 28-day intermediate option during the pandemic. It has a more expensive, fully-refundable option that lets you cancel until very shortly before you're due at the hotel, which is more generous that many hotels allow for their flexible reservations. I don't remember whether PI's flex rate requires cancellation the day the reservation begins or the day before. What's different from most other hotels is the existence of the intermediate, 28-day option.

I only book something less than the most flexible option when I'm already in Europe and within a very few days of the planned arrival date, and even then only if the savings are worthwhile. I'd rather pay more and not have to worry about a change in plan. However, for plenty of hotels that most flexible option still ties you down once you are within 72 hours or so.

The flex rate is cancellable until 1pm on the day of arrival

I always choose the flexibility rate with the same day 1PM cancellation for the sane reasons as others have stated. For me that is peace of mind and worth the extra cost. With flight cancelations,strikes, covid, etc. anything can happen to disrupt your plans.
And of you are adding non credit card travel insurance you are paying something extra for those non refundable reservations to be completely covered.

Sandra, Premier Inns usually have 3 options although there are actually 5 options they offer overall. You can read about the various options in more detail at their website: https://www.premierinn.com/gb/en/why/rates.html

They range from payment upon arrival with free cancellation up to the date of arrive, to payment up front with NO refund and NO cancellation or changes. Obviously the prices for the most flexible will be the highest.

I usually use the 2nd option (the one you asked about). I don't mind paying up front, but do like to have the option to cancel even if it's 28 days out. I just did a dummy booking for a Premier Inn in Devon, and the most flexible one cost £140 for 2 nights, the "intermediate" (which you asked about) was £115 for 2 nights, the the least flexible was only £1 cheaper at £114 for 2 nights. Sometimes their pricing makes no rhyme or reason. But definitely pay attention to the pricing and choose the one that works best for you.

I've been booking Hiltons lately and when it's time to pay, you are offered several different cancellation options at differing prices. You choose your comfort (and budget) level. I think being locked into reservations these days is pretty risky.

Thanks, everyone, for your replies.

I'm pretty sure I will be staying at the same place I've stayed the last several times I was in London, but I thought it might be a good idea to have a backup (or two) in case something goes wrong. I'm sure I'm not alone in no longer believing that the future will unfold the way I expect it too! I think that's been the strongest holdover for me from the last three years. Of course, the future never was written in stone...