I take on board the comments about not going, and we have agonised about this.
We've decided to go, and I know this may appear selfish, but I will explain in partial mitigation:
The Tier system is a broad brush using arbitrary boundaries and we live in an area that finds itself in T3 due to being in a wider geographical region but doesn't have a high infection rate (lower than quite a number of places in T2).
Without wanting to unfairly brand groups of people, we all know there is a huge variation between those that adhere to all precautions and those who either refuse or can't be bothered to do so. We are all scrupulous about following procedures and don't visit any higher-risk places like pubs or gyms.
On a more selfish note, the kids look forward to this so much. I call them kids, they're teenagers, and have missed out on so much this year (like everyone). The eldest daughter was born on Halloween so this time of year has always been that bit more special. She missed out on prom and all the other school-leaver stuff. This may be the last time she wants to do this with her family (she'll be 18 next year; the years just fly by!). This is, I think, our 8th consecutive SF (and we've had a couple of bad years, ruined by 'that certain group' having a mass day-out) and will be a rare treat.