I dont consider myself properly experienced at trackdays, i've only been to 2-3. my experiences seem to be different than what some say.
first, brake fluid - upgrade it. I use ATE typ200 which can be got at a decent price from euro car parts with their various sales. near as doesnt matter, same temp to motul RBF600 which is the de-facto fluid for track use.
second, brake pads. ime, fast road pads (and thus OEM) wont cut it on the rear end. seems to contradict what experiences other have had however. Get something that can handle the heat - Dixcel Z, pMu HC800's, pagid RS29's etc (all 700-800C temp). things like the projectMu NS (around 450c iirc) will just turn to dust on track.
discs will be fine. mine have had a hammering and no issues with them but i suspect once they're worn out, it'll be beneficial to go high end discs down to lifespan.
tyres - i run dunlop RT2's and wore down maybe 1.5mm at most on 1 track day. 1 trackday and around 10k road and they still have 4mm left on them. the conti 6's are made of cheese so keep and eye on them or take a second set. Most seem to be recommending ps4s, and i'm planning cup2's when the RT2's expire.
events: i've only used MotorSport Events (aka MSE) that run airfield days. They're about the cheapest events around (abingdon is £130) and i've not found any stupidity going on at these events. plenty of room for error so excellent for new guys. you'll need a helmet (they rent 'em at a small cost) at bare minimum. driver training - try to get to the event early to get booked in with an early timeslot.
if your set on a race circuit day, check to see if they are allowing pro drivers on as this seems to cause issues for a few no-pro attendee's. a few thread on Pistonheads reguarding it and doesnt seem the best environment for new guys.
Insurance: i'm with A-plan as they give 5 trackdays/yr for £250 plus £1000 track excess as long as you get road insurance with them. They were the lowest i could find and came recommended. Ad-hoc cover seemed to be around that £200 mark you've found. You could try morris (
https://trackday.moris.co.uk/) as when i was looking, they were not able to provide a quote due to some in-house changes they were doing.
miscellaneous stuff: dont worry about timing or speed. focus on being smooth, predictable to other drivers and your awareness of other drivers. Speed will come at a later time. Also, if your feeling intimidated or flustered or frustrated with others, come into the pits. i've been on track with new people who seem to be unaware of faster cars it became frustrating behind behind them. theres nothing to gain but everything to loose, so come into pits, calm down and by the time you go out again, they'll be off the track or else-where on the circuit away from you.
afaik, any UK event wont allow suction cup mounts, if you do, they gotta be secured by another method to stop them flying round the car when they come unstuck. Bolted, screwed, clamped systems are ideal.