If your flight was booked to the UK as a single ticket then, as the UK remains for now in the EU, EU law applies.
The law says that any flight originating and/or terminating within the EU comes under the strictures of the law. The law says that the airline must offer a refund, not a voucher but a refund. I am not entirely clear but I think they can offer you an alternative flight but if you turn that down you are still entitled to a full refund of the originally paid fare. I am pretty sure that you can find the full wording and/or an "essential points" of the law on various sites if you google "EU passenger rights". There are other points involved, such as delays, etc., but you will at this point be most interested in cancellation.
If your original travel back was a composite of two or more flights then the law only applies to the last one/part. If it was a single ticket of connecting flights but with the same airline then there may just possibly be some wiggle room for the airline on everything but the last one - get advice, there is a unit in the UK (the CAA I think) which processes claims and gives advice. If it was a case of separate airlines then you only have a claim against the last one. Airlines did not like the law when it came out (no surprise) and did various things to thwart it, others dislike it simply because it is an EU law. Ryanair famously ended up in court and lost, others still try to fob people off with vouchers.
My advice to you at this point would be to refuse the voucher(s) and clearly state in writing that you want the matter dealt with in accordance with EU law (in the wording watching out for the risk of cutting yourself off regarding any non-EU flight). On return to the UK, approach the claims authority with your claim and request for advice.