In reply to the uncomfortable truth:
There are very few freelance climbing instructors that don't have a handful of other feathers in their caps and other outdoor skills/quals. Never had an application from anyone with a climbing qual that doesn't also have other outdoor certificates - kayaking, daingy sailing, mountain biking, archery...etc. etc.
Think about centre work vs. freelance and which would suit you better, the standard issue route for freelancers is an aweful lot of work at the start to get established, gainging experience at walls/activity centre, low pay, long hours, childminding type work (e.g. harnessing kids, orienteering games, nature walks etc. as apposed to leading someone up tower ridge in your first week), gaining contacts, spending a lot of your wage on training and spending large amounts of time travelling around the country. A lot of outdoor instructors in their 30's and 40's are trying to find ways to get away from the instability of freelance work - working towards being senior staff, mangement, doing more office work related to the outdoors, going down the training route etc. So there would be a few painful years for you I guess but there's no reason you couldn't work towards having some stability and a stable £20kpa in 5-10 years time depending on what route you took. Outdoor instructions isn't going to make you money, give you a pension or put your kids through uni.
It might be useful to brainstorm how else you can market your skills to support your work as a climbing instructor and expect that the amount of climbing work you do in your first few years will be more of a treat and not your main source of income while you're getting qualified in various other disciplined. Even then a lot of your time will be spent belaying and watching other's do well at easy climbs compared to climbing yourself. How are you going to get by in the winter? Can you use a chainsaw, work at heights, drive a digger, fix boats, work in a climbing shop, do maintenence work, have decent seo/web development experience, groundswork/gardening? Most of the instructors I've come across do those kinds of things, even MIC's with 30 years on you.
Just as an observation, outdoor instuction is customer service, make sure that's what you want to do. People are paying for your personality and spending time with you a lot more than they are your climbing skills. All things being equal, an enthusiastic people person with experience leading groups and entertaining screaming kids that can only manage to scrape up a VS is going to have a lot more potential than an e2 cruiser who's climbed all their life but isn't a people person. Also goes without saying you're probably going to want to live close to outdoors hotspots.