Best weather in mid-March—-Cornwall?

The general trend is that that the west side of the country is wetter than the east, while the south is milder than the north.

Having said that you might get a storm coming in from the Atlantic and battering Cornwall, yet it’s a balmy spring day up in Northumberland.

Mid march is still (just about) winter so don't expect particularly nice weather anywhere in the UK. It also can be rainy anytime, anywhere in the UK.
It is true that the south is normally warmer than the northern areas but again this cannot be guaranteed. The eastern side of the UK is normally drier. Cornwall is stuck out in the Atlantic so gets the brunt of the prevailing weather. Wales is also in the west.
Shropshire whilst being western is in the shadow of the Welsh mountains so often has a drier micro climate than you expect.
Yorkshire also benefits from being sheltered by the Pennines and I often find it to be dry and sunny when I travel over from the west side.
Northumberland is beautiful, but a long way north and winter tends to hold on there a little longer.
Typical Mid March weather would be 8-14C down to 2C at night. Rain anytime. Two years ago in March we had "the beast from the east" which brought bitterly cold weather to the UK, not much fun for walking.
From your list, I would suggest Yorkshire. Plenty of walking options in the dales and moors. Plus the option of visiting places like York, Leeds, Bradford, Sheffield, Harrogate, Yorkshire Mining Museum, Meadowhall, peak district and much more if the weather is poor or just if you fancy a change.

Timeanddate.com has actual, day-by-day historical weather for St. Ives going back at least ten years. I didn't check for other locations. I'd take a look at the last 5 years at least.

This website has some averages that might be useful, but there may be better sources for those. I note that the Wikipedia entry for St. Ives doesn't include the usual weather-summary chart. At a glance, I'd say rain falls on an average of about 1 day out of 3, but I'll certainly defer to the UK experts.

This is very helpful and I thank you.

We would rather be cold than wet, so I will look at the options in Northumberland and Yorkshire.

Now I see Isle of Wight is also an option with HF Holidays. Might this be somewhat more sheltered from the Brunt of Atlantic storms than Cornwall?

Not really more sheltered, the two storms that we just had recorded winds of 100mph at the Needles (a chalk rock formation on the west of the I.O.W.) with buckets of rain.

I really wouldn't base your holiday choice on the off chance of better weather in one place over another. It's just too variable.

Go with your first choice, take a coat and a sense of adventure.

OK! Two more: Exmoor, near Selworthy, and Peak District, near Thorpe.

I have checked current weather and 14-day forecast for both, and theynare similar: a mix of rain days, partly cloudy, and sun, about 1/3 each, with dailynhighs of 6-9 degrees. And I know this is not necessarily accurate this far ahead.

What about wind? Seems like the Exmoor location could be windy. Is it known for that in early spring?

March is a fickle month..
Not long lasting but it often snows first week in April in the Peak District ," lambing snows"
This news horrified my neighbour's when they moved up from Cornwall as they often saw no snow in winter.

Not really more sheltered, the two storms that we just had recorded winds of 100mph at the Needles (a chalk rock formation on the west of the I.O.W.) with buckets of rain.

The Needles always record high wind speeds but these drop considerably on the rest of the island and on the mainland. Whilst the Needles can be experiencing 100 mph winds, 24 miles away in Portsmouth it could be 40 mph. The IOW in March is a good bet. if you're going to experience a storm you're going to experience it to some degree or another in much of the UK but March isn't much of a stormy month anyway. The Isle Of Wight is a much underestimated island, an excellent place to visit with much to see and do, the only downside is the hefty price tag to get there!

Thanks again. I apologize for sounding so scattered in my approach. We just last night decided to cancel the Italy portion of our March trip ( Venice and MIlan) and spend more time in the U.K. We already have 5 nights in London and want to get outside for some nice walking. A number of HF Holiday options fit our dates perfectly, somwe just need to decide.

We have good rain gear and cold-weather clothing, so I guess we will not worry about weather, and choose the two that appear most appealing in terms of terrain and scenery. And try to keep it within 3 hours by train from London, which rules our St. Ives.