Doing gardening work (ie. digging / landscaping / constructing rather than hoe / mow / blow) is hard work physically and tends to be expensive for the client.
Garden design / build budgets can quickly mount up to astronomically silly numbers.
I think £100 to get this area sorted ready for seeding is OK. That the guys worked fast, or that the area turned out to be smaller than perhaps what they thought, or that they did it quicker than you anticipated is really neither here nor there, sadly.
From the sounds of it, they turned up on time, they did the job (fast, and hopefully well) and they've finished / tidied and gawn....
I'd certainly suggest you pay them and you get on with planting the seeds to you can benefit from some attractive lawn ! One less thing on your to-do list and it's not even midday !!!
In the future, you could enquire about an hourly rate, and perhaps work like that (you may find things take, ahem, a little longer than you'd have hoped though !)