Welcome to the forum
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1. Does the ukba care how long the relationship has been for or do they care
More about the fact that it is legitimate?
They care more that the relationship is legitimate - they need to make sure the marriage isn't forced or a sham just to get a visa.
So, you need to provide your marriage certificate along with a couple of photos of you together, plus evidence of regular communication between you for the duration of the relationship.
Things like:
- boarding passes from visits to see each other
- letters/cards you might have sent each other
- a screenshot of your email inbox showing regular communication by email (just a list of dates and subject lines)
- a screenshot of your Skype (or similar) call log to show regular calls.
2. Our plan is to marry in the registrar office and then have a big wedding down the line. This is mainly to be with eachother quicker. Will having a registrar wedding be a red flag to the ukba?? Do I need to explain this to them ?
No, it's not an issue at all - lots of people have done a similar thing.
You can't legally marry twice anyway, so only your registry office ceremony will be relevant. Your second wedding can only be a blessing ceremony, not an official wedding and if it's going to be later down the line, there's no need to even mention it in your application at all.
The application is about ticking boxes (genuine relationship, financial requirements and suitable accommodation) and showing hard facts - UKVI don't care about the details of your wedding ceremonies or how much you love each other, just that you have a legal document showing you are married and your relationship is not forced or a sham.