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There are at least 3 ways you might be able to tackle this. Starting from expensive and low work and moving on to cheaper options with more work, you could consider:
1) Mesh networking kit: You buy a pack of mesh base stations (BT, Google, other) and put one in each cottage. This should require no drilling or anything, but is potentially expensive. BT's Whole Home WiFi costs £200 for 3 discs and provides one network over the properties. Other solutions exist like Google Wi-Fi, Netgear Orbi, etc. You should look for reviews and see if this might work in your situation. The BT option seems like good value for money, albeit not the fastest.
2) Wi-Fi extension using HomePlug. If the cottages are on a common electrical circuit you could use a wi-fi extender solution. One approach would be something like BT's Mini HomePlug 600 which combines a wi-fi base station with a HomePlug adapter in one unit and carries ethernet frames over the existing powerlines. Variations on this would be using plain homeplug adapters and installing a wireless router in access point mode. Pretty much any wireless router would do this for you, I'd go for one with external aerials as they can be slightly more flexible in controlling the area covered.
3) Run one or more ethernet cables from the house to the cottages and have wi-fi routers attached to the ends. You'd configure the additional wi-fi routers as wireless access points (1-click configuration). Personally, I'd run external cat5e for this as it's cheaper and easier to work with than cat6 upwards and will carry 100Mb/s which is plenty for a cottage. If you go this route, I'd probably install ethernet sockets in the cottages and wire those up for flexibility. You can pick up an ethernet cable tester for ~£10 that'll help check the wiring is correct. This isn't too hard to do, but you probably want an SDS drill for going through the walls. You could stick a wifi router in access point mode in each cottage with the power on each router configured a bit low to minimize interference.
Feel free to ask questions about any option here, just sketching answers above. Others here might have different approaches too.
One more option for you if you can get ian ethernet cable from the house into the crawl space in the roof of the cottages. You can buy power-over-ethernet wifi access points (many options, e.g. TP-Link EAP115). This will include a power-over-ethernet adapter which you'd plug in inside the house and then run 20m of ethernet cable along the ceiling and ceiling mount the wifi access point(s) in the cottages. This would make it easy for you to turn on-and-off from the house and makes it hard to tamper with. The downside is that the configuration of the router is a bit trickier than using a regular wi-fi router as an access point or using HomePlug based wi-fi extenders.