I'd expect that your escallonia will have a bit of a problem in such a location, for at least two reasons.

Firstly, it is difficult for anything to compete with the shallow pig of a root system that many maples have. Not only will your new seedling be in stiff competition for water, but for physical space under the soil, as well.

Secondly, this plant should probably be planted in as sunny a location as possible in London. The rather heavy shade of a maple tree all during the growing season will make it even more difficult for the escallonia to get enough direct sunlight.

Could you turn it into a tub plant for a few years at least?

rhizo is absolutely right about the maple roots. They're hungry and too close to the surface. Something like Nandina copes pretty well, and surface-rooting plants such as Lamium.

Escallonia is hardy, evergreen, and useful as a hedging at the seaside. Easily trimmed and has nice flowers. Any room at the front door for a very pleasant tub-grown evergreen?