Nice pair of pistols.
The post-war Mark 1 was available in both nickel and chrome plated versions from Webley, but they did not offer these options with the Premier. Even so, there are a few plated Premiers about, which would have been non-factory modifications carried out by individuals.
How fussy are you about getting the exact keeper screw? You could round off a 7BA cheesehead bolt in a Dremel and leave it in the polished state. While not an exact fit, 7BA will screw into the Webley thread a fair way and give you a sound fix. It would certainly look better than showing a blued screw head.