Thanks for being patient while I did some digging.
First, I think because you aren't a huge spender you need a flat rate. This is going to make payments predictable and you can really start to value the work being done and trust the manager of the account. Your money will go toward working for you, rather than being micromanaged at the bid level.
Second, when you are calling around and making decisions about who to work with be aware of their internal structure. If you were my client at Distilled, I would be your point of contact and I would be the one working on your account. I think you should look for a company with an internal structure where you will also have this direct path for communication because you know you are getting pure, unadulterated information on your account. 
Recommendations::
The moment you've been waiting for!
Because I think it's so important that you have a flat rate, I'd suggest GetSquare.co.uk, I spoke with a trusted, knowledgeable friend here who said "for small spend accounts we generally charge a fixed monthly management fee that covers optimisation, management & reporting." They're located up in Edinburgh, if location matters to you.
Another alternative to Anicca would be Boom-Online.co.uk. I also know first hand that there are knowledgeable people here who you can trust. They are located in Nottingham. They do a review, then off the back of the review price a one time set up fee, then it's a flat fee based on spend & number of campaigns.
Whatever you do, be sure you feel you are getting the most out of your preliminary conversations with any agency you are working with.
Let me know if this information has been helpful, or if you have any lingering questions!
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I'm sorry that looks so shitty)