Secret Flying for flight alert emails (free to sign up, but emails are daily once per day so only use if you don't mind daily emails).
Google flights ( www.google.co.uk/flights ) on your phone for price tracking alerts, so you can set up alerts and get notified and book when the price drops. Skyscanner for ideas / route checking by month or specific dates using 'Everywhere' function.
Complete Savings cashback site for 10% cashback on hotel and airline sites (mostly main carriers though not budget airlines) and holiday companies. Always book direct with airline rather than some third party OTA you have never heard of just to save a few quid. For accommodation, check price direct as booking sites charge up to 25% commission (usually 15% minimum base commission and then more if they want to be featured, appear higher in the search results etc). Always worth a quick email or phone call to accommodation to see if they will beat booking site price or throw in an extra like free breakfast or room upgrade, although many people can't be bothered to do this for some reason hence the popularity of the booking sites. Sites like booking dot com useful for finding places though (use the filter function and sort by function) and for reviews (unlike TripAdvisor only people who have made a booking and stayed there can leave a review) but keep in mind their rating system misleadingly starts at 2 not 1 or 0, so an overall rating of 8/10 is more like a 6/10.
Hotels dot com as mentioned gives you one night free for 10 booked, although more limited selection of places you can book your 'free' night. Can be combined with CS cashback so effectively up to 20% discount.
Ebookers gives you 10% off the first booking made using their app, but check cashback sites too, eg topcashback currently doing 12% cashback on them.
Use a cashback credit card to book accommodation if payment is by card, usually only 0.5% but every little helps, but make sure it's a credit card that charges no fees for overseas transactions and uses Visa or MasterCard exchange rate as accommodation will almost always bill you in the local currency. Can combine with Curve card and app to pay for flights and avoid credit card fees on those and still get points/cashback.
Take Starling / Monzo / Monese card for spending abroad with no fees and best exchange rate + fee free ATM withdrawals (limited to 200 GBP/EUR per month with Monzo, no limit with Starling, not sure about Monese).