@irina are you going to Bologna? Fancy meeting for a coffee there? I will be at the fair the whole week.
I’m not familiar with the UK agencies (though I know Bright on their fame alone) and @NessIllustration has given you plenty of advice from her awesome experience.
Your work is excellent and you’re more than ready to get picked up by an agent - indeed I think you can have your pick! An agent can open doors that would be very hard to tackle by yourself...but it comes at a cost, of course. For me it was totally worth it, also because I know I wouldn’t want to spend too much of my time in promotion mode (it´s already plenty, even with an agent....).
Keep in mind that a Literary Agent (representing book illustrators) will cost much less than an Illustration Agent (representing all kinds of illustrators) - 15-20% of all contracts instead of 35-50% of all contracts. You can choose a mixed setup, like I did, where you have a literary agent for book work and you take care of all other types of work by yourself. Nowadays, I think I would like to have an agent take care of all the other stuff too, so I’ll be looking into getting a second rep for non-book work next year.
As for day job, I have a 20-hour contract as art director for a small internal corporate comms agency (the corporation is big, the internal agency is small ;-)). I love it because I get to do illustration there too, as well as working with other super-talented artists and designers: I learn loads of stuff that comes in very useful also in my children illustration business. It´s also very well paid. So, since you worked in architecture, do not discard the possibility to work in the same field in a different role. I hear there is an appetite for hand-drawn renderings or sketches of spaces and not all is 3D. I’ve also been involved in designing exposition booths as part of my day job, and there are artists there too - and agencies that churn out ideas for those using illustration and sketches rather than 3D software. Do not underestimate the value of your expertise and look for creative ways to position in for other jobs.
I’d personally always favor a job that is close to illustration and has a strong intellectual involvement rather than a more „classic“ day job, like being a shop assistant or waiter. The first is a source of learning and inspiration and is generally more profitable. The alternative is often just a time-sink (though I’m sure you can get loads of inspiration from working in contact with people in all cases).