Nope. It had been above water for at least 120,000 years prior...probably longer.
You’re talking about the duration of the ice age itself. The plain was exposed during most or all of the ice age, but it was probably inundated during the preceding interglacial as well. There’s no geologic evidence to indicate any massive uplifting in the area over the past million years or so, making it a fairly safe assumption that the North Sea plain has been at or near its current elevation since the Alpine uplift began tens of millions of years ago. I’ll admit that I haven’t studied that region specifically, but from what I understand it hasn’t been above water permanently for millions of years.