hello and welcome the GMB

I suspect that technically your decanter isn't an antique, in the usual sense of being more than 100 years old - however a very nice piece, and one that deserves to be filled.
It is the acid backstamp showing the Union Flag which shows this to be a C20 piece - it would seem that their mark incorporating the flag started some time in the 1920's, and continued in use after they were taken over by T/Webb and then on until around the 1960/70 period, I think, although not too certain of the finish date. This one might be anywhere during that entire period, and a rough idea of when might be indicated by amount of wear, internal staining and whether the cutting is sharp or not. But your bottle is most definitely Richardson.
You should find a corresponding '2' somewhere on the lip or neck of the body, assuming this is the matching original stopper - stoppers often go missing and are replaced with substitutes although these rarely fit well, and then the Nos. either don't match, or one part lacks a No. completely. These Nos. don't say anything other than that, if identical, then probably both parts started out life from the factory together.
Richardson glass is perhaps less common than much other C20 glass, and I wouldn't expect you to readily find another with the same design. In view of the lack of damage/wear, I'd suggest this one was made post 1945, and shape wise possibly a 'Prussian' - a shape common in the C19, and copied widely since.
If you go on line and punch in Richardson, you should find quite a lot of information regarding the mark and other history of the company.