As with anything, you should never entrust your money to an organisation without first making sure they are legitimate. While international money transfer companies may not have the same Financial Services Compensation Scheme protection that banks have, there are some key things to look out for.
The main indication of a legitimate, trustworthy service, in any country, is customer reviews. If there are enough other people telling you that they have used it successfully and liked it, then you can be confident that you too can trust it (just watch out for paid-for reviews! If a review sounds too good or generic to be true, it might just be). In the UK, there is another key indicator, namely whether the transfer company is authorised by the Financial Conduct Authority (FCA).
The FCA is the UK's financial watchdog, and businesses offering payment services are required to be either authorised by or registered with the FCA. If a company is authorised, your cash is protected so that you should be able to get it back if the firm finds itself in difficulties. If it is registered instead, as smaller companies tend to be, your cash may not be safeguarded, as these companies can choose whether to comply with the safeguarding rules.
However, sending money abroad using an authorised service should be just as secure as sending it to another bank account within the UK.