To add to the point above: having shot architecture in those cramped city spots for years, I found that zooms are often more practical than primes. Switching lenses on the fly when you have a wedding lined up is just asking for dust on the sensor. Here is how my experience shakes out:

• The Canon RF 14-35mm f/4L IS USM is fantastic for flexibility. You get that extra wide reach, and the stabilization is a lifesaver for handheld interiors. It is pricey, but you save time in post.
• The Canon RF 15-30mm f/4.5-6.3 IS STM is the budget alternative. It is not as sharp at the corners as the L-series, but for digital real estate work, it gets the job done without draining your savings for wedding gear. Basically, if you need the crisp edges for high-end prints, go L-series. If you are mostly doing web-based architectural work, the cheaper zoom is fine.

TL;DR: If you want perfection for architecture, grab the zoom, but if cash is tight, the prime wins for now. Basically, the Canon RF 14-35mm f/4L IS USM is an engineering marvel! The sharpness across the frame is insane, and the constant f/4 aperture makes indoor architectural shoots so much more consistent. It is pricey, but the build quality is totally worth it for professional work. If you really need to save for that wedding season, look at the Samyang AF 14mm f/2.8 RF. It is surprisingly sharp for the price point and gives you that ultra-wide view without the L-series tax. You miss out on the zoom versatility, but for tight London interiors, 14mm is just fantastic. Honestly, the 14-35mm is the pro choice, but don't sleep on that prime if you have a budget limit!

Honestly if you are doing architectural stuff in London, the distortion on some of the cheaper glass can be a nightmare to fix in post. I have been using the Canon RF 16mm f/2.8 STM for a while now, and while it is not L-series glass, the price is unbeatable for what it offers. You might want to consider the following before dropping heavy cash on the zoom:

• The 16mm prime is tiny so it barely adds weight to your bag when you are moving between wedding venues.
• Make sure to check your lens profile in Lightroom because it has some aggressive barrel distortion at the edges that needs correction.
• If you really need that 14mm reach, look at the Canon RF 14-35mm f/4L IS USM but only if you can find it used, as the retail price is pretty steep. Be careful with the f/4 aperture on that zoom though, especially if your wedding work involves darker indoor receptions. You might find yourself cranking that ISO up way higher than you want. I would suggest picking up the prime first to see if 16mm hits the spot for your architecture work. If you find you are constantly swapping lenses, then maybe save up for the zoom later. Just dont blow your entire budget on the lens right before the busy season starts, cause you will definitely need that extra cash for batteries or better lighting gear.