Ok, I'm going to break your question into a few parts.
1. Should you break your fixed rate for Eur 329?
You're currently paying 3.75%, with 18 months left in the term.
UB will allow you to break this for 329, or an annualised 0.18% over the remaining term.
If you switch to the loyalty discount rate of 3.2%, you'll be saving 0.55% per year
Over 18 months, that's a a saving of 685 (1,014.75-329). I'd say go for it.
It's very interesting to see an actual breakage quote, and aligns with what other posters (including
@Sarenco ) have been saying about breakage costs - they must be tied to market rates, and as markets have been relatively flat for the past 2 years breakage costs are very low at the moment. It might be worth while for people on higher fixed rate to investigate what their break costs would be...
2. Should you pay 21k off your mortgage early?
If you do, you will save 672 interest in your first year (assuming you've already switched to 3.2% rate).
If you continue with your plan to repay your mortgage in 8 years, with compounding you'll save yourself 6,018 in interest, just from that single early repayment. To repay the remain balance over 8 years would be approx 1,206 per month.
However, don't leave yourself short! Repay other loans first, have money set aside for emergencies, and make sure you're not missing out on employer matched pension contributions.
3. Should you fix your rate, or move to variable?
If you have a look around the forum you'll find much debate about the current high variable rates, and whether there will be pressure on banks to reduce rates. Personally I'd say fix if you want absolute certainty, but otherwise stay on the discounted variable for now and have the flexibility of paying off lump sums whenever you want.
4. Should you look at a switcher option to get a better rate / cash back?
Lot's of advice elsewhere on this.