It really depends on the upgrades. If it involves restocking and adding iron sights, I think you are better off starting with a Model 70 etc. with sights and an acceptable stock. The Whitworth actions are solid, but I would state they are what they are—solid entry level rifles. I know they have been utilized by some talented smiths for very nice builds, but this is not commonplace.
I don’t follow Whiitworths, but I do not recall seeing one (in .375 etc) without iron sights. They could be commonplace, as again, I don’t follow them, but I certainly don’t remember seeing one. If you like synthetic stocks and want an entry-level rifle, I don’t think this is a bad choice at all. I just think there are better starting points if you’re trying to do something nice.