Good type of bike to get for commuting (thinking of road v touring)
There are a bunch of bikes sold in the UK that are specifically designed for the cycle-to-work scheme: they have good specs for commuting, are priced to come in under the £1000 limit, etc. A number of online vendors stock these. So that would be a good place to start. Or find someone selling one of those bikes used.

The additional weight of a steel bike is not that significant. Less than 1 kg. When you get done fitting mudguards, racks, lights, a toolkit, and whatever you're schlepping to work, you won't really notice the difference. That said, as Argenti points out, you'll probably be shopping for either steel or aluminum. They're both good options.

Also budget for the clothing and accessories you'll need to ride comfortably and safely. That would be a subject for another post entirely.