Hi Ian,
Good to see a fellow countryman here 😊

As for your question
Inviting a lady from Ukraine is way simpler, because they do NOT need a visa as long as they have a biometric passport. (If they say otherwise, they're scammers and please do report them!)
Russia is a little more tricky, but still easy:
Of the five existing types of visas we can comfortably ignore
- diplomatic
- working
- student
visas
That leaves you with two options:
1) a regular tourist visa
(Including full board and tour)
2) a visiting visa
Inviting people to the UK is a nightmare process yet still doable, if you plan carefully and well ahead.
Otherwise I most strongly recommend booking a cheap (and lousy) package tour - which are really within an easily affordable range, considering that they arrange everything (visa, invitations, return 'guarantees' and insurance).
You can always meet your lady and take her some place else, where you'll be on your own.
Good luck and the best of success for your journey and meeting!