It depends on which price a broker is choosing to display as their default one. Each instrument has three prices:
1) Bid (SELL)
2) Ask (BUY)
3) Trade (Last Trade Price)
If brokers choose to utilise market data of the same type, such as Ask, they should be equal if they receive quotes from the primary exchanges of the instrument. However, some brokers may opt not to work with the primary exchange due to price constraints and instead choose smaller alternative exchanges that may offer less favourable prices, resulting in slightly different Bid, Ask and Trade quotes.
The Invest/ISA charts display only the Last Trade Price providing information about the most recent transaction for the instrument and serve an indicative purpose.