Registration for VAT is compulsory once the turnover exceeds £85,000 in a rolling year
VAT is chargeable on supplies of goods or services and can be recovered from purchases of goods and services in most cases.
There are complex rules for trading with overseas entities, and there are specific rules for selling to overseas customers whether business to business (13213) or business to customer (B2C). Professionals such as accountants and solicitors are always B2C.