Edit: Appears I was totally wrong regarding the transfer and sale as in option 3. As noted but the other commenters, CGT will be due on the market value at the date of gifting and the rate will be based on your income (20% after the 12,300 allowance).
I've left the original answer below anyway.
I'm not sure if there are particular rules regarding the disposal of crytocurrency, I'd highly recommend checking with an accountant/tax lawyer who has a background in crypto before acting.
In terms of tradtional assets however, the tldr is:
if you transfer the assets directly, you may benefit from him paying 10% less CGT in the difference between his current earnings and the higher rate band (50,250), however given the complexity (and potential lawyer fees) this may cause with getting the repayment to the lender approved, it may be easier to gift cash.
Before doing any of this, I'd strongly recommend having a chat with an accountant/financial planner rather than just some guy on the internet.
The capital gain
In option 1, you would pay the capital gains, as a higher rate taxpayer, you'd get the first £12,300 tax free, but thereafter pay a rate of 20% on the gain.
In order to finance the loan in option 2, the capital gain will be the same, so there will be no change there.
As noted by AakashM in the comments, unless there is a formal agreement prepared detailing a repayment plan, the loan will fail the 'Duck Test' and will be treated as a gift for all intents and purposes.
Under option 3, the gift is transferred tax free, so no capital gain is payable on transfer. The asset is valued at the base value you bought it for however: in this case either your purchase price or 0 if you mined the asset originally.
Your partner would then sell the asset, as he is a basic rate payer, he will pay intitially at 10% (following the same 12,300 allowance).
Your capital gains however are added to your total income for the year when calculating your tax band. Since the value of the mortgage is likely to exceed the basic rate tax band, he will pay 20% on the proportion of the gain that exceeds the basic rate band (in addition to income).
https://www.gov.uk/capital-gains-tax/rates
Inheritance Tax
Inheritance tax will only be a consideration if you were to die within 7 years of the transfer. If you were to die in this period, this transfer would be assesible in your total estate, but the rate of tax may be reduced depending on the date of death.
More info on these rates can be found here: https://www.gov.uk/inheritance-tax/gifts
Under either option 1 or 3, the value of the transfer would be the 'Market Value' of the goods/cash at the date of transfer.
i.e. you would not pay inheritance tax on the purchase price of the crypto at the date of transfer, but the market value on the day.
If your partner delays before selling for the cash to repay the mortgage, the assessible amount will still be the value on the day of transfer, which introduces a further risk.
As noted before, if there was no formal arrangement regarding the loan, this would likely be treated as a gift and assessed accordingly.
If there was a formal agreement/repayment plan, the cash itself would not be taxable, but the loan agreement would specify to whom the repayment obligation was due in the event of your death.
If all assets were to pass to your partner, he would effectively inherit the legal title to receive cash, which would then factor into the estate received, resulting in no net difference in inheritance tax to just giving the cash originally.
Note also that as of 2021 inheritance is tax free up to £500,000 (if a property is included of a value >= £125,000) so if the assets are below this value, no tax is payable.
https://www.gov.uk/government/publications/inheritance-tax-nil-rate-band-and-residence-nil-rate-band-thresholds-from-6-april-2021
Lastly, tax is not a good reason for marriage/civil partnership, but inheritance passed between married/civil partners is tax free.
Money Laundering
If your partner is intending to repay his mortgate using gifted cryptocurrency and/or the profits of cryptocurrency, be aware that there will be serious scrutiny applied by the lender to avoid money laundering.
I would recommend ensuring that you have seriously detailed records and evidence prepared regarding: the date & purchase price of the currency, the date & sale price of the currency, any intermediary transfers.
At most lenders, even a cash gift used for repayments requires an attestation letter from the giftor often signed off by a lawyer.
I imagine that a gift transfer of cryptocurrency used before the repayment will set off all sorts of alarm bells at any responsible lender.