Making use of Both ISA allowances for UNMARRIED Couples

My partner (of several years, joint home owners, and joint current account holders) does not have enough spare funds to take full advantage of their own (£20k) ISA allowance for this tax year. I have sufficient funds to fill both of our ISA's this tax year. Can anyone explain the best legal route to get my £20k into their ISA without either of us incurring subsequent income tax liability as a result, e.g by exceeding the annual £3k gift allowance? A link to relevant HMRC guidance would be useful. (Scottish tax payers FWIW) e.g. Can I just transfer £20k into our joint current account and they transfer the £20k into their ISA from there?

Hi Baggio,

There are no income tax implications on the receipt of a cash gift unless the cash gift generates interest or dividends.
These would then potentially be subject to tax.  Further guidance can be found here Tax on savings interest and Tax on dividends.   
There are no tax implications on the giving or receipt of cash gifts, but you may wish to speak to inheritance tax regarding any inheritance
tax implications How Inheritance Tax works: thresholds, rules and allowances and Inheritance Tax: general enquiries.  
You may also want to review the guidance at How Inheritance Tax works: thresholds, rules and allowances. 

Thank you.