Thanks for your patience.
Provided that you are able to evidence your relationship during the time that you are in the UK and she is in Columbia then you should be fine. So, you would have to document your travel there by keeping evidence of your flight tickets, receipts and photographic evidence of the visit for both of you.
If you are to open a joint bank account in Columbia then this would also help hugely.
Your other evidence can be evidence of how you have kept in contact when you are not together. So, things like emails, evidence of phone calls, text messages would be fine.
The reality is that if the UKBA see that you have maintained your relationship for two years prior to actually applying for the spouse visa then they won't have any doubts of the credibility of the relationship.
She is free to apply for visit visas before and after you are married, but you should be aware that the UKBA may be a bit suspicious (particularly after you are married) that she may intend to seek permanent stay in the UK whilst on the visitor's visa which is strictly forbidden. Therefore you would have to produce considerable evidence of the continuing ties to Columbia which would compel her to return there at the end of the visitor's visas.
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