I see the benefits of sowing from seed as cheaper than buying plants.
Being able to grow a few bits in winter.
Any more benefits ?
If you don't mine the faff of raising plants, rather than sowing direct, I think there are some benefits:
Plants are reasonable size when planted out. The weeds are always one step behind (compared to direct sowing)
Earlier crop if you can start tender things off a bit early
Exact numbers, no gaps in the rows. I start 4 Cauliflowers off each fortnight, and half a dozen lettuce. (No neat rows here though ... looks like a patchwork quilt!)
Downside is Faff (time, plus you need a supply of pots, and have to be around to water the "babies") and some cost (multi purpose compost - but that does get added to the soil via the rootballs, so I justify it as "soil conditioning")
Is a polytunnel better than a green house ? And can anyone recommend good ones to buy that are reasonably priced ?
I prefer a glass house - but they are hideously expensive by comparison. Mine came off eBay, second hand, and were cheaper than a new same-size polytunnel would have been - they come up on Freecycle too.
You allotment may have a policy on no-glass, or no-plastic for that matter ... glasshouses that fall down / yobs chuck stones though, are a nightmare of glass chards for future allotment owners, and abandoned plastic that has deteriorated into small pieces is not very pleasant either.
Only downside is the need to attend it regularly - watering plants, and opening in the morning to let air in, and closing at night (you can have automatic openers for Glasshouse vents - and you can rig up unattended drip irrigation for either) - that's obviously easier if the greenhouse is in your garden, or the Lotti is not far away - and if you don't go away a lot!