I'm in similar boat in some respects and have planned and started fabrication on my first custom from scratch build, having done installs and mods to off the shelf hilts before now.(Look forward to sharing in near future when it's come along a bit)
I've actually been keeping a log of expenditure and breakdown of hours spent and categorizations for this (research, design, process planning, sourcing parts and materials, tests, fabrication) which might seem a bit much, but has taught me a lot that's really helpful to apply to second build - useful knowledge that I'd be much less able to apply if I was doing 2 simultaneous builds. If I had a lot of free time and was looking at days or a short number of weeks, this wouldn't be a big deal, but defo glad to be doing one at a time where it's going into weeks and months. It also gives me an idea of time and costs if I look to making sabers to sell in future. A plus of working on more than one saber I can see is that, if making more than one part (or basis for a part), I've found a much larger proprtion of time than I anticipated has been in setting up work holding and tooling on my small lathe and mill for a particular part or operation, so something to be said for making multiples of parts there.
I've also been learning a lot that I just didn't anticipate until I got to starting the actual fabrication and found oversights in my planning, which has resulted in a bit of avoidable use of time and metal and offers lessons for next build.
Like you, I'm in the UK, and ordered a bunch of aluminium from Aluminium Warehouse - ordering a good amount offsets the cost of collection (if you're nearby like me) or delivery. This also means that I have the opportunity to work on simpler sabers if I have to put my main build on hold for a bit (and simple doesn't have to mean crappy - take the Kota design for example - pretty simple, but looks awesome).
So from my experience, and echoing some of what others have written, I'd recommend sticking one relatively complex/ambitious build so you can carry what you've learnt to the next build and doing some 'playing' on simple hilt designs if you have spare time or time gaps waiting on parts during your main project. Certainly when the build involves a lot of learning and new processes.
Of course, that's just putting forward some of the pros of what works for me and there's a huge personal aspect to it. Worth considering how you manage to juggle more than one project in other contexts like work, study, family, etc. I tend to focus on one thing a bit obsessively and give it my best when circumstances allow and don't work so well or enjoy when I'm working on more than one thing simultaneously, but others thrive on this, so that might (or might not) suit you better.