Probably your two options if you want to ride both on and off road are a gravel bike or a hybrid. Gravel bikes are basically road bikes, with drop handlebars and clearance for wider tyres, but they have no suspension. They tend to have lower gearing than road bikes, but not as low as mountain bikes. So they are good for riding on forest tracks/fire roads etc but not really for more serious off road riding. They're fine on roads, especially if you swap the tyres from the knobbly off road ones they usually come with for the smooth road type.
Hybrids tend to have flat bars and you can get them with front suspension which would be potentially useful for your forest rides. Gearing again is somewhere between road and MTBs and you'd have the same options with tyres. If you buy a bike with suspension, make sure that it is lockable as you will probably want to lock it off if you're riding on smooth roads. A hybrid will almost certainly have a more upright riding position than a gravel bike, which you may find more comfortable if you haven't ridden a bike for a while.
Really its a matter if personal taste and what style of bike you think you'd find more comfortable. Another factor is how hilly your area is. If there are hills then you want to go for the lightest bike you can get, with nice low gears, but if you are in a flat area that's less important.
As with many things, you do get what you pay for with bikes, but you can get something that would be a reliable every day bike for between £500 and £1000. Obviously at that price point you aren't going to get a super lightweight carbon fibre frame or top of the range components but a lot of modern aluminium frames and cheaper components are still very good.
I would agree with the advice to avoid Halfords, except for the Boardman range. There are some very capable bikes in that range, and not badly priced. Decathlon's own brand B'twin often get good reviews when cycling magazines do "best budget buys" articles, though I have never ridden one myself so I can't give a personal opinion. People can be snobby about these kind of bikes, and its true, you wouldn't enter the Tour de France on one. But since you don't plan on doing so, that doesn't matter does it?! The best bike is one that fits you, is the right sort of bike for the terrain and that you enjoy riding. Don't worry too much about the brand name.
However, if you want to spend a bit more, I'd probably go for something like this www.canyon.com/en-gb/hybrid-city/touring-bikes/pathlite/
I'm a big Canyon fan - they are very nicely made bikes. The only downside is that they only sell direct, there are no dealers, so you can't usually try before you buy. The online fitting tool is pretty good though. Or at least its worked for me.
Lots of brands have similar models though, at around the same kind of price. As has already been mentioned, you might be able to buy through a cycle to work scheme, and some schemes will also cover additional costs like helmets, locks etc.
I'd start by visiting a few different shops to get an idea of what seems to suit you. Unfortunately you may find there is not a lot in stock at the moment though. This is usually a great time to buy a new bike as shops are keen to clear out old stock to make room for the new season's models so you can often get good deals in the Autumn. However, Covid has put paid to that this year, both because most 2020 models are sold out and because production of some 2021 bikes has been delayed. So you may find the shelves are a bit bare at present I'm afraid. I've ordered a new bike this week but have no idea when it will arrive. I hope its before Christmas but they couldn't promise even that.
Good luck with your hunt - I hope you find something that suits you and that you enjoy your rides.