New debt issuance calendar ( how much monthly / annual)
"This calendar includes all the key operational dates including gilt auction dates, the dates when auction sizes (and for new gilts the coupon) are announced and major redemptions. It also includes dates associated with the implementation of the remit e.g. quarterly consultation meetings, dates of quarterly gilt auction (and Treasury bill) issuance calendar announcements, and, when known, the dates of the Budget and Spring Statement."
EDIT: for your question 4, see the "market notices" section of the BoE site. E.g. for the current month (Feb 2021), https://www.bankofengland.co.uk/markets/market-notices/2021/february/asset-purchase-facility-gilt-purchases-february-2021. Has info on what maturities they plan to buy, in what quantities, and on which dates/times.