They may arrest some people still, they will probably lie about who people are or what everyone wants, but united we are invincible. They cannot arrest an entire country, or even a tenth of it. If anyone has been arrested or punished in any way for refusing to pay towards the deliberate murder of children, the tax-strike will not end until they are released and pardoned.
Taxes will not be "back-payable", this is not a negotiation for better salaries or tax rates or any complicated political doctrine that requires a nuanced democratic debate. This is a simple demand to STOP ARMING ISRAEL. To stop spending UK taxpayer's money directly OR indirectly on any military support for what has been categorically proven to be genocide. Many of us will want them to go further, to oppose Netanyahu beyond Great Britain or to support aid convoys, but for the sake of our unity and to ensure the solidarity of everyone, the demand for now will simply be to stop aiding Israel.
You will simply cancel your direct-debits, stop your standing-orders, and keep as much council, corporation and personal tax as you're able, until the UK renounces all support for Israel's genocide and commits to provide no further military, intelligence, material or financial support to them.
If you choose to hold that tax money and send it to HMRC after the government has complied, that will be up to you, however the UFP will call on everyone to support everyone's right to have simpy kept their taxes due during the strike.
If the government tries to intimidate us by using our love of the institutions they have neglected against us, by refusing to pay NHS staff, we will then ask that everyone in the UFP send the taxes they would be paying to somewhere detailed on this website here, which we will send to our nurses, doctors and essential support staff, to ensure the continued (probably better paid) running of essential services.