Hi @theL8investor welcome to the T212 community. The most important thing to remember with ISAs is always initiate a transfer and never be tempted to sell and withdraw the cash to move it to a new provider. Doing that would count as adding new money in the tax year and contribute towards your ISA allowance.
If you follow the two options you mentioned this will not count as adding new money to an ISA so you will either be able to use the full £18k to buy new funds offered by T212 or if you buy VWRP you could transfer than in Specie. Just make sure whatever you do, you initiate a transfer to T212 and dont withdraw any cash.