Both herself and my son didn't know it was wrong to enter the country on that VISA for seeing each other (as well as doing some sightseeing). It seems it is quite strict that you come into the Country for the exact and only reason of your VISA?
It's not wrong to use that visa - it's exactly the right (and only) visa they can use to see each other, you just have to be careful using it and you need to make sure you have evidence of being a genuine visitor and proof you will not try to overstay... because whether you are allowed into the UK or not depends entirely on the immigration office (IO) at the airport.
If it looks like she's trying to visit the UK too often (spending more time in the UK than in Lebanon) or she is using a visitor visa to 'live' in the UK, they can refuse entry... and she's not alone - entry refusals happen every day.
Some tips for visiting the UK successfully:
-try to spend at least as much time outside the UK as you did inside before you come back... so if she stayed for 4 months, then left, she should try to wait another 4 months before coming back
Each time you try to enter:
- have evidence of a return flight
- have bank statements showing you have enough money to support yourself for the entire trip without working
- have evidence of ties to the country where you live, which mean you HAVE to go back, such as:
1) letter from employer giving you time off and stating when you will return to work
2) evidence of a home you rent or own back home, which you are maintaining while in the UK
3) evidence of other commitments you must return for (family members, interviews, appointments, events like weddings etc.)
- show awareness of the visa rules and that you know you have to return to your country of residence to apply for a fiancé visa
So, just because she was refused entry this time doesn't mean she has to apply for a fiancé visa earlier than planned... it just means that if she tries to come back as a visitor again before applying for the fiancé visa, she should apply for a visitor visa in advance and make sure she has evidence that she will leave the UK and not try to stay.
So she could either apply for the fiancé visa sooner than planned and move earlier, or she could wait and instead try to get an advance visitor visa to come back for another visit.
There is also the Marriage Visitor Visa option which is much cheaper than the fiancé visa (£87 compared with £1,195) and would allow her to marry in the UK as a visitor then return to Lebanon to apply for a spousal visa to move to the UK after that - thereby saving £700+ in visa fees.
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