Can I voluntarily register for VAT below the threshold?

Yes, you can register voluntarily at any turnover level, and it's a common choice when most of your customers are VAT-registered businesses, since they can reclaim the VAT you charge them and it won't increase their costs. The main benefit is being able to reclaim VAT on your own business expenses and setup costs; the trade-off is the extra admin of filing returns and, if you sell mainly to the public, potentially having to raise your prices. Whether it's worth it depends on your customer base and cost structure, so it's a good one to talk through with an accountant.