Hi Adam,
I think your concern to get an agent is perfectly understandable, because the acting marketplace these days is so competitive, and the amount of decent paid work open to you without an agent is pretty limited. While having an agent is not, and should not, be thought of as the be-all and end-all of developing a successful, or at least rewarding, acting career, it shows a professionalism and maturity in your approach that you are eager to start off in the business as you mean to go on.
What you say about waiting on good showreel material is sensible, because it is true that many of the best agencies will want to see that you have notable credits guaranteeing your saleability before they wish to consider taking you on, and your showreel will be the tool that can show them that. Yet, it's unlikely that, even if they ask you to come back to them when you have a showreel available, most agencies would write you off simply because you didn't yet have one.
Your mainstream TV credit (won, I assume, from your own efforts at canvassing) and your regularity of working since first coming into the market both speak well of your potential. In the eyes of many agencies, this is the best advantage you can have, and it may well be worth attempting to write to big agencies already - the worst that can tend to happen is that they send you back a refusal letter. There is also no guarantee that any refusal now would be a refusal later down the line - your perceived status or saleability within the industry can change as you age, fashions change and you are seen to offer different possibilities to casters.
Having had a look at your profile, though, I would say you have a classic 'leading man' potential, which is always in demand. You are tall, a good looking guy from your headshots, your ethnicity is very castable (and very 'now' - this may seem a strange thing to say, but you have to begin to think as casting directors think). You are also young, which can sometimes be a disadvantage in the marketplace (as there are no more oversubscribed areas within the industry), but is also often a benefit - as it's assumed you have untapped potential, and it may be well worth a mentor or two taking you on now so that in later years they can say they helped make you into what you will become.
From this perspective, I think you have as good (if not better) a chance as most to get some significant agencies to take an interest. It may be possible for you to whet their interest without pushing matters too far by sending out feelers and telling them that you would rather be considered seriously when you have the showreel available, but, in the meantime, would they be interested in a meeting? Again, you can't know until you try.
The bottom line will always remain: if they think you are going to be an advantage on the agency books, an agency will always chase you, regardless of whether you have a showreel or not!! It's all worth a punt!
I appreciate w