I would just stick to free spins offers if you've only signed up to 7 bookies. There are over 50 decent sign up offers that make money easier and risk free instead of casinos which except the free spins offer do involve risk.
The different slots have different levels or RTP (return to player) and variance. So a slot with an RTP of 95 that is low variance would return £95 for every £100 wagered. On average. A high variance slot with an RTP of 95 would return £950 of every £1000 wagered.
Casinos are not risk free. They are more advantage play. And whilst some people make really good money from them, there are also downswings of busting out or losing £10 15x in a row. So unless you have a larger bankroll, it's not a good idea to risk some of it.
Free spins are always good to do as big wins can come from them occasionally and also they are completely risk free. Today I've had £15 from 3x10 free spins offer. So they are really worth doing. But until your bankroll is larger and you've done most of the sign up offers, you could lose most of your money with no return.
MissFitton
Here is a list of bookies that offer regular low-medium risk casino offers I do
William Hill often do 'bet £10 on certain slot, get 10 free spins' the 30 seconds of free spins ones aren't really worth doing as you can only really get 8ish spins and the slots are usually low rtp and the EV is low.
William hill have one at the moment which is 'wager £10 and get £5' the £5 has wagering on it. It has an EV of about £3 and a good tactic is to wager the £10 on blackjack using wizard of odds strategy and then taking the £5 bonus to a high RTP slot and wagering any winnings from it.
Dunder do 'deposit x get y free spins on certain slot' again you wager your deposit on blackjack and then withdraw it, and then do your free spins and if you are in profit after your spins and wagering, cancel your withdrawal and then withdraw it all.
Virgin Games and Star Spins do a wager £5 on 3 days and they give you a cash box bonanza. You can wager the £15 on blackjack if you don't want to risk it or do it on slots and hopefully get a good win. The cash box bonanzas have been brilliant for me. I've had over £300 in the last 2 weeks from them.
Jackpot joy also do box bonanzas
Grovesnor casino do free spins daily most of the time and a good 'wager £10 get 10 free spins'
I have a list someone gave me that I use, I can find it at attach it if people want? It's a very comprehensive list of most of the low risk offers out there.