Yes. You can hold as many ISAs as you like, and since 6 April 2024 you can pay into more than one of the same type in the same tax year, provided your total subscriptions stay within the £20,000 allowance.
Moving an existing ISA from one provider to another is a transfer, and it does not use any of this year's allowance. Always use the provider's transfer process: withdrawing the money and paying it in elsewhere turns a transfer into a fresh subscription and burns the allowance.
Money paid in during the current tax year has to be transferred in full if it is transferred at all. Money from earlier years can be moved in part.
Withdrawing the money and paying it in elsewhere turns a transfer into a fresh subscription, and burns your allowance.
No. Plenty of people accumulate several over the years. The practical argument for consolidating is that it is easier to see what you have and what it costs.
It can, in platform charges, and it makes the whole harder to manage. It can also help, where different providers suit different jobs.
Contact the provider or HMRC. Excess subscriptions are usually unwound and the returns on them can become taxable.
No, provided it is done as a transfer through the providers. Withdrawing and re-paying does count.
Most people accumulate several over the years and lose track of what is in them. Bringing them together is usually part of a first review.
A first conversation of about twenty minutes, at no cost to you. Tell us what is on your mind: a pension you have lost track of, a fixed rate ending, a will you keep meaning to write.