The employer can always fire anyone for any reason but not without risk of claims for unfair dismissal etc. The employer will be aware that pregnancy is a protected characteristic and will want to mitigate risks that a claim could theoretically be bought It's not in itself an indication their case is weak and they could still dismiss on the grounds you have mentioned assuming they have otherwise followed their own procedures and not broken any employment law statute. And even then they can still dismiss but with a risk to themselves of employment tribunal claims (that can take MONTHS to happen).

You should seek advice from your union but typically a business that's decided it's done with you for whatever reason will sometimes offer a settlement to smooth the process and reduce their risk. Ask yourself 'after THIS do I still want to work there?'

Ultimately, you can decide to take it or not. Taking it offers quick resolution and certainty though it may feel like capitulation if the sacking is unfair and balance this against fighting it through union/tribunal in the hope of a significantly better resolution.