Commute + tow bike
 

I commute about 25 miles a day so I need something reasonably fast (able to sit comfortably at 18mph without a ridiculous amount of effort). I also need something that can be used to tow my son around woods/country parks etc. in his trailer at the weekends. I did have 2 bikes, a road bike and a hybrid which worked pretty well, but unfortunately some f***** stole them both from my garage last weekend. Could both of my bikes be replaced with a gravel bike or should I stick to my 2 bike formula? How rough can you go with these gravel bikes before you need a hybrid? I haven't ruled out an electric bike but my only issue with them is they're restricted to 15mph (in the UK at least). This is a bit useless for my flat commute, so I'm not sure how much assistance it will actually offer, but it would be a bit more rugged for the weekends I guess? Any advice would be gratefully received before I make an expensive mistake. Thanks.

Just about any average bike can tow a trailer. If you want a flat out race bike then you might check if the gears are low enough, or if the manufacturer says not to do it.

If I had the same situation, I would look at a gravel style bike to go with the trailer. I commonly use my Burley Travoy for Grocery store runs and use a combination of many different bikes to do it, including a road bike, gravel bike or even a single speed. A gravel bike is the best mix since it should have a large range of gears and a more aggressive riding posture, plus room for bigger tires. It seems that the 650B (27.5 MTB) is popular these days, so you can change out tires as needed for terrain and that's what I would do.

I'd stay with 2 bikes formula, sometimes some bikes (gravel?) are designed to do lots of things, pavement? off-road?, etc? but are never that great at any!

I'd get one dedicated tool that does one job right and does it well. besides I'd have some redundancy if for some reason one bike has some mechanical problem,

A gravel bike will definitely do what you describe very comfortably. In my case, I keep my old MTB-to-hybrid bike for towing, because it's great to use an older/beat up bike for this. The number of times that bike has fallen over / scratched, because I need to attend to my kid in the triailer... :) Honestly, it's great to use a bike that you don't baby for stuff like this.
If you do decide to go for a gravel bike, I'd recommend having a closer look at gearing. Broadly speaking, there's gravel bikes with two types of cranks - road cranks (Tiagra, 105, Ultegra), and GRX / third party (eg. FSA) cranks. The latter tend to have smaller chainrings, which are key to towing.