The numbers generally sound good.
UK is famous for being overcast. How much sun you get is key.
Since you'll get several times as much sun in summer as winter, panels tilted for max winter production probably works best.
No mention of charge controller? Select SCC and design PV array (series/parallel) to fit each other. Voc of series PV string must never exceed max input voltage of SCC. You can calculate exactly using Voc of PV panel, temperature coefficient, and record cold temperature of your location. Or multiply Voc by 1.16 for a safe estimate.
More PV is always good. 5 x 260W is a good amount of power for your need, but can only be configured 5s or 5p. If you can get six, 3s2p and 2s3p give additional voltage options. With multiple panels or series strings, I like multiple panels (or series strings) oriented toward morning and afternoon sun to flatten curve of power production.
I usually suggest powering freezer during the day only and not at night to get by with small battery, but since you expect multiple days without sun that doesn't help.
Your surge current measurement sounds accurate. I've measured other motors (not refrigerator) with scope, and see about 5x nameplate rating. 67W running is nice and low. 685 Wh/day is about half a full-size fridge sold in the U.S.
50% and 25% DoD indicate lead-acid. If 3 days without sun is rare, you could go to 75% on that rare occasion. If power outages are rare you can cycle deeply. I use AGM for my whole-house backup system, with 70% DoD low-battery disconnect. AGM or Gel is less work than FLA (no maintenance) but doesn't last as long.
Lithium gives more cycles, but costs proportionately more unless you DIY, which can cost just 40% of AGM to purchase. But DIY and LiFePO4 have various issues to deal with. For this size bank and fewer than 300 discharge cycles over a decade, I don't think it is worthwhile.
Here is the battery I use, 6V 405AH AGM, so two for about $625 each. You may find similar brands locally.
The Sun Xtender Battery line consists of specialty lead-acid solar, photovoltaic batteries, and renewable energy storage batteries manufactured by Concorde Battery Corporation.
sunxtender.com
Here's one of the best FLA
rollsbattery.com
You will be limited to what is actually available with reasonable lead time, due to current market conditions. One forum member was going to get SunXtender but was many months before delivery, so he bought Rolls that was in-stock.
Either type of Lead-acid, but especially FLA, prefers a specific charge current. For FLA, probably 0.12C or 0.15C. Oversize PV array and an SCC that limits current to that target charge rate would be good. Important that SCC has battery temperature sensor and a good charge profile. For FLA, check cell specific gravity periodically and give it an equalizing charge when needed.
If you have an inverter already, probably doesn't have AC transfer switch. When power fails you can move power cord to inverter and turn inverter on. Or rig up automatic transfer switch. Inverter running continuously? That drains battery a bit at night, cycling batteries unnecessarily. Could automate remote on/off switch. Or have a float charger plugged into grid power to keep it charged at night.
Oops, we forgot something: inverter no-load drain, which could be as much as refrigerator's 67W but 24/7, never turns off. If 50W, then 1200 Wh/day, twice the refrigerator's 658 Wh/day. Look up no-load consumption. If possible, measure. A 12V timer could turn inverter on/off by its remote switch (if it has one), so only operated during 12 daylight hours, cutting drain in half. Freezer needed about 10 hours operation (685W/67W), but my guess is it runs even fewer hours, lower kWh consumption, in cool weather. Wire an extra freezer thermostat to inverter remote on/off switch?
Need to recalculate battery capacity to cover inverter no-load consumption and hours of operation per day.
I like the idea of thermostat turning inverter on/off best. Set slightly colder than freezer's thermostat so inverter only runs when freezer does. (note that if refrigerator/freezer, thermostat is in refrigerator and freezer runs colder based on air valve.)