Are you sitting on some unspent Stocks and Shares ISA allowance for this tax year? Any allowance unused before the end of 5 April can’t be carried over to 2025/26. So it may be worth using up as much of that £20k yearly allowance as possible before it’s too late.

Investors don’t have to actually purchase any shares, trusts or funds before the deadline to shelter their money from tax. But given the cheapness of many London Stock Exchange-listed assets, it may be a mistake to delay.

With this in mind, here are two top FTSE 100 and FTSE 250 bargain shares I think investors should consider today.

Greggs

Not even its focus on value foods and treats has saved Greggs‘ (LSE:GRG) bacon in recent times. Sales have slowed considerably in recent times, and remain in danger of further weakness in the current economic climate.

Yet I believe the cheapness of its shares makes it worth a close look. Its forward price-to-earnings (P/E) ratio of 13.1 times sits comfortably below the company’s five-year average of 20.8 times.

Many of the long-term drivers that pushed its market-cap from £1bn in 2015 to £1.8bn today remain in place. Most critically, further store additions to supercharge sales are in the works, with up to another 150 planned this year alone as the baker moves closer to its 3,000 outlet target.

There’s also much more room for growth in the white-hot delivery segment. Sales from this channel increased 30.6% year on year in 2024 as the company extended the service to 1,556 outlets.

With Greggs saying this month it enjoyed “improved trading in February“, investing in the FTSE 250 firm before the next market update on 20 May could be a good idea to consider. Though there’s no guarantee that sales haven’t deteriorated again following last month’s uptick.

Ashtead Group

Like Greggs, rental equipment supplier Ashtead Group (LSE:AHT) also looks cheap from an historical perspective. Its prospective P/E ratio is 15.7 times, some way under the five-year average of 21.1 times.

There’s good reasons why the company — which operates under the Sunbelt brand — now commands a much cheaper valuation. Weak construction markets in the US and Canada have seen it sharply downgrade near-term sales and profits forecasts. They could continue to deteriorate too as the threat of crushing trade tariffs hits North American economies.

But there’s also plenty to remain optimistic about. The FTSE 100 company stands to benefit greatly from a series of mega Stateside infrastructure projects planned over the next decade. It could also gain from significant onshoring in the US and Canada if trade wars intensify.

Ashtead’s rolling expansion drive puts it in great shape to exploit its positive long-term market outlook too. The firm’s market share in the US is 11% today, up from 6% a decade ago. But there’s substantial room to increase this through organic investment and acquisitions in what is a highly fragmented marketplace.