Let me take a look at this and I'll get back to you in a bit.
If you let a property at less than the commercial rate, you may still make a profit in a tax year and if you do that will be taxable income. Expenses are deductibe up to the level of the rent and any loss cannot be used in later tax years. Take a look here for information on property not let at a commercial rent. You should disclose the income and expenses in a tax return annually. You will be entitled to the personal tax allowance to offset against any profit you make. You can register for self-assessment by completing a form SA1 here.
You may have to pay CGT if and when you sell the property, notwithstanding the fact that it is your main home or will be if and when you live in it. Take a look at Example 9 in HS283 here where a house has been the main home of the owner and it has been let.
I hope this helps but let me know if you have any further questions.
I can't help you with your membership query I'm afraid. You should contact just answer customer services.
If you sell your property to a connected person, then market value will be used if the property is sold at less than its true value. That doesn't apply to rental income.